diff --git a/nw/config.py b/nw/config.py
index 3149edef..94c30f71 100644
--- a/nw/config.py
+++ b/nw/config.py
@@ -20,7 +20,7 @@ from os import path, mkdir, unlink, rename
 from datetime import datetime
 
 from PyQt5.Qt import PYQT_VERSION_STR
-from PyQt5.QtCore import QT_VERSION_STR, QStandardPaths
+from PyQt5.QtCore import QT_VERSION_STR, QStandardPaths, QSysInfo
 
 from nw.constants import nwFiles, nwUnicode
 from nw.common import splitVersionNumber
@@ -156,6 +156,10 @@ class Config:
         else:
             self.osUnknown = True
 
+        # Other System Info
+        self.hostName  = QSysInfo.machineHostName()
+        self.kernelVer = QSysInfo.kernelVersion()
+
         # Packages
         self.hasEnchant  = False
         self.hasSymSpell = False
diff --git a/nw/constants/constants.py b/nw/constants/constants.py
index 4cc7f040..b39830bc 100644
--- a/nw/constants/constants.py
+++ b/nw/constants/constants.py
@@ -23,6 +23,7 @@ class nwFiles():
     APP_ICON    = "novelWriter.svg"
     PROJ_FILE   = "nwProject.nwx"
     PROJ_DICT   = "wordlist.txt"
+    PROJ_LOCK   = "nwProject.lock"
     SESS_INFO   = "sessionInfo.log"
     INDEX_FILE  = "tagsIndex.json"
     OPTS_FILE   = "guiOptions.json"
diff --git a/nw/guimain.py b/nw/guimain.py
index a9cd7edb..a34f8f28 100644
--- a/nw/guimain.py
+++ b/nw/guimain.py
@@ -15,6 +15,7 @@ import time
 import nw
 
 from os import path
+from datetime import datetime
 
 from PyQt5.QtCore import Qt, QTimer
 from PyQt5.QtGui import QIcon, QPixmap, QColor, QKeySequence
@@ -49,9 +50,9 @@ class GuiMain(QMainWindow):
         self.hasProject = False
         self.isZenMode  = False
 
-        logger.info("OS: %s" % (
-            self.mainConf.osType)
-        )
+        logger.info("OS: %s" % self.mainConf.osType)
+        logger.info("Kernel: %s" % self.mainConf.kernelVer)
+        logger.info("Host: %s" % self.mainConf.hostName)
         logger.info("Qt5 Version: %s (%d)" % (
             self.mainConf.verQtString, self.mainConf.verQtValue)
         )
@@ -319,7 +320,42 @@ class GuiMain(QMainWindow):
 
         # Try to open the project
         if not self.theProject.openProject(projFile):
-            return False
+            if self.theProject.lockedBy is not None:
+                if self.mainConf.showGUI:
+                    try:
+                        lockDetails = (
+                            "<br><br>The project was locked by the computer "
+                            "'%s' (%s %s), last active on %s"
+                        ) % (
+                            self.theProject.lockedBy[0],
+                            self.theProject.lockedBy[1],
+                            self.theProject.lockedBy[2],
+                            datetime.fromtimestamp(
+                                int(self.theProject.lockedBy[3])
+                            ).strftime("%x %X")
+                        )
+                    except:
+                        lockDetails = ""
+
+                    msgBox = QMessageBox()
+                    msgRes = msgBox.warning(
+                        self, "Project Locked", (
+                            "The project is already open by another instance of %s, and is "
+                            "therefore locked. Override lock and continue anyway?<br><br>"
+                            "Note: If the program or the computer previously crashed, the lock "
+                            "can safely be overridden. If, however, another instance of %s has "
+                            "the project open, overriding the lock may corrupt the project, and "
+                            "is not recommended.%s"
+                        ) % (nw.__package__, nw.__package__, lockDetails),
+                        QMessageBox.Yes | QMessageBox.No, QMessageBox.No
+                    )
+                    if msgRes == QMessageBox.Yes:
+                        if not self.theProject.openProject(projFile, overrideLock=True):
+                            return False
+                    else:
+                        return False
+            else:
+                return False
 
         # project is loaded
         self.hasProject = True
diff --git a/nw/project/backup.py b/nw/project/backup.py
index bab5ffcc..1f2bd985 100644
--- a/nw/project/backup.py
+++ b/nw/project/backup.py
@@ -57,7 +57,9 @@ class NWBackup():
         baseName = path.join(self.mainConf.backupPath, archName)
 
         try:
+            self.theProject._clearLockFile()
             make_archive(baseName, "zip", self.theProject.projPath, ".")
+            self.theProject._writeLockFile()
         except Exception as e:
             self.theParent.makeAlert(
                 ["Could not write backup archive.",str(e)],
diff --git a/nw/project/project.py b/nw/project/project.py
index 887fe38c..afde1e02 100644
--- a/nw/project/project.py
+++ b/nw/project/project.py
@@ -40,6 +40,7 @@ class NWProject():
         self.projOpened  = None # The time stamp of when the project file was opened
         self.projChanged = None # The project has unsaved changes
         self.projAltered = None # The project has been altered this session
+        self.lockedBy    = None # Data on which computer has the project open
 
         # Debug
         self.handleSeed = None
@@ -178,7 +179,7 @@ class NWProject():
 
         return
 
-    def openProject(self, fileName):
+    def openProject(self, fileName, overrideLock=False):
         """Open the project file provided, or if doesn't exist, assume
         it is a folder, and look for the file within it. If successful,
         parse the XML of the file and populate the project variables and
@@ -201,6 +202,21 @@ class NWProject():
         if not self._checkFolder(self.projMeta):
             return
 
+        if overrideLock:
+            self._clearLockFile()
+
+        lockStatus = self._readLockFile()
+        if len(lockStatus) > 0:
+            if lockStatus[0] == "ERROR":
+                logger.warning("Failed to check lock file")
+            else:
+                logger.error("Project is locked, so not opening")
+                self.lockedBy = lockStatus
+                self.clearProject()
+                return False
+        else:
+            logger.verbose("Project is not locked")
+
         try:
             projectMaintenance(self)
         except Exception as E:
@@ -308,6 +324,7 @@ class NWProject():
         self.setProjectChanged(False)
         self.projOpened = time()
         self.projAltered = False
+        self._writeLockFile()
 
         return True
 
@@ -398,14 +415,19 @@ class NWProject():
         self.mainConf.updateRecentCache(self.projPath, self.projName, self.currWCount, saveTime)
         self.mainConf.saveRecentCache()
 
+        self._writeLockFile()
         self.theParent.setStatus("Saved Project: %s" % self.projName)
         self.setProjectChanged(False)
 
         return True
 
     def closeProject(self):
+        """Close the current project and clear all meta data.
+        """
         self._appendSessionStats()
+        self._clearLockFile()
         self.clearProject()
+        self.lockedBy = None
         return True
 
     ##
@@ -645,6 +667,73 @@ class NWProject():
     #  Internal Functions
     ##
 
+    def _readLockFile(self):
+        """Reads the lock file in the project folder.
+        """
+
+        if self.projPath is None:
+            return ["ERROR"]
+
+        lockFile = path.join(self.projPath, nwFiles.PROJ_LOCK)
+        if not path.isfile(lockFile):
+            return []
+
+        try:
+            with open(lockFile, mode="r", encoding="utf8") as inFile:
+                theData = inFile.read()
+                theLines = theData.splitlines()
+                if len(theLines) == 4:
+                    return theLines
+                else:
+                    return ["ERROR"]
+
+        except Exception as e:
+            logger.error("Failed to read project lockfile")
+            logger.error(str(e))
+            return ["ERROR"]
+
+        return ["ERROR"]
+
+    def _writeLockFile(self):
+        """Writes a lock file to the project folder.
+        """
+
+        if self.projPath is None:
+            return False
+
+        lockFile = path.join(self.projPath, nwFiles.PROJ_LOCK)
+        try:
+            with open(lockFile, mode="w+", encoding="utf8") as outFile:
+                outFile.write("%s\n" % self.mainConf.hostName)
+                outFile.write("%s\n" % self.mainConf.osType)
+                outFile.write("%s\n" % self.mainConf.kernelVer)
+                outFile.write("%d\n" % time())
+
+        except Exception as e:
+            logger.error("Failed to write project lockfile")
+            logger.error(str(e))
+            return False
+
+        return True
+
+    def _clearLockFile(self):
+        """Remove the lock file, if it exists.
+        """
+        if self.projPath is None:
+            return False
+
+        lockFile = path.join(self.projPath, nwFiles.PROJ_LOCK)
+        if path.isfile(lockFile):
+            try:
+                unlink(lockFile)
+                return True
+            except Exception as e:
+                logger.error("Failed to remove project lockfile")
+                logger.error(str(e))
+                return False
+
+        return None
+
     def _checkFolder(self, thePath):
         if not path.isdir(thePath):
             try:
diff --git a/tests/reference/proj/2_nwProject.nwx b/tests/reference/proj/2_nwProject.nwx
index b78c14a4..0a8e6b4d 100644
--- a/tests/reference/proj/2_nwProject.nwx
+++ b/tests/reference/proj/2_nwProject.nwx
@@ -1,5 +1,5 @@
 <?xml version='1.0' encoding='utf-8'?>
-<novelWriterXML appVersion="0.1.4" fileVersion="1.0" timeStamp="2019-06-08 20:53:03">
+<novelWriterXML appVersion="0.4.5" fileVersion="1.0" timeStamp="2020-02-26 22:05:41">
   <project>
     <name></name>
     <title></title>
@@ -72,28 +72,28 @@
       <paraCount>0</paraCount>
       <cursorPos>0</cursorPos>
     </item>
-    <item handle="39fa9ec190eee" order="None" parent="None">
+    <item handle="8722616204217" order="None" parent="None">
       <name>Timeline</name>
       <type>ROOT</type>
       <class>TIMELINE</class>
       <status>New</status>
       <expanded>False</expanded>
     </item>
-    <item handle="d029fa3a95e17" order="None" parent="None">
+    <item handle="96061e92f58e4" order="None" parent="None">
       <name>Object</name>
       <type>ROOT</type>
       <class>OBJECT</class>
       <status>New</status>
       <expanded>False</expanded>
     </item>
-    <item handle="81b8a03f97e87" order="None" parent="None">
+    <item handle="eb624dbe56eb6" order="None" parent="None">
       <name>Custom1</name>
       <type>ROOT</type>
       <class>CUSTOM</class>
       <status>New</status>
       <expanded>False</expanded>
     </item>
-    <item handle="da4ea2a5506f2" order="None" parent="None">
+    <item handle="f369cb89fc627" order="None" parent="None">
       <name>Custom2</name>
       <type>ROOT</type>
       <class>CUSTOM</class>
diff --git a/tests/test_project.py b/tests/test_project.py
index c2f4078d..6723dcc7 100644
--- a/tests/test_project.py
+++ b/tests/test_project.py
@@ -29,6 +29,7 @@ def testProjectNew(nwTempProj,nwRef,nwTemp):
     assert theProject.newProject()
     assert theProject.setProjectPath(nwTempProj)
     assert theProject.saveProject()
+    assert theProject.closeProject()
     assert cmpFiles(projFile, refFile, [2])
 
 @pytest.mark.project
@@ -41,9 +42,21 @@ def testProjectSave(nwTempProj,nwRef):
     projFile = path.join(nwTempProj,"nwProject.nwx")
     refFile  = path.join(nwRef,"proj","1_nwProject.nwx")
     assert theProject.saveProject()
+    assert theProject.closeProject()
     assert cmpFiles(projFile, refFile, [2])
     assert not theProject.projChanged
 
+@pytest.mark.project
+def testProjectOpenTwice(nwTempProj,nwRef):
+    projFile = path.join(nwTempProj,"nwProject.nwx")
+    refFile  = path.join(nwRef,"proj","1_nwProject.nwx")
+    assert theProject.openProject(projFile)
+    assert not theProject.openProject(projFile)
+    assert theProject.openProject(projFile, overrideLock=True)
+    assert theProject.saveProject()
+    assert theProject.closeProject()
+    assert cmpFiles(projFile, refFile, [2])
+
 @pytest.mark.project
 def testProjectNewRoot(nwTempProj,nwRef):
     projFile = path.join(nwTempProj,"nwProject.nwx")
@@ -59,6 +72,7 @@ def testProjectNewRoot(nwTempProj,nwRef):
     assert isinstance(theProject.newRoot("Custom2",   nwItemClass.CUSTOM),    str)
     assert theProject.projChanged
     assert theProject.saveProject()
+    assert theProject.closeProject()
     assert cmpFiles(projFile, refFile, [2])
     assert not theProject.projChanged
 
@@ -96,6 +110,8 @@ def testIndexScanThis(nwTempProj):
     assert str(theBits) == "['@tag', 'this', 'and this']"
     assert str(thePos)  == "[0, 6, 12]"
 
+    assert theProject.closeProject()
+
 @pytest.mark.project
 def testBuildIndex(nwTempProj):
     projFile = path.join(nwTempProj,"nwProject.nwx")
@@ -117,3 +133,4 @@ def testBuildIndex(nwTempProj):
     assert theIndex.buildNovelList()
     assert str(theIndex.novelList)  == "[[1, 1, 'Novel', 'SCENE'], [3, 2, 'Chapter', 'SCENE'], [5, 3, 'Scene', 'SCENE'], [7, 4, 'Section', 'SCENE']]"
     assert str(theIndex.novelOrder) == "['31489056e0916:1', '31489056e0916:3', '31489056e0916:5', '31489056e0916:7']"
+    assert theProject.closeProject()