diff --git a/pcbnew/drc/drc.cpp b/pcbnew/drc/drc.cpp
index cc0a9f7aa4..3748e02ea4 100644
--- a/pcbnew/drc/drc.cpp
+++ b/pcbnew/drc/drc.cpp
@@ -61,7 +61,8 @@ DRC::DRC() :
         PCB_TOOL_BASE( "pcbnew.DRCTool" ),
         m_pcbEditorFrame( nullptr ),
         m_pcb( nullptr ),
-        m_drcDialog( nullptr )
+        m_drcDialog( nullptr ),
+        m_rulesFileLastMod( 0 )
 {
     // establish initial values for everything:
     m_doPad2PadTest     = true;         // enable pad to pad clearance tests
@@ -98,7 +99,7 @@ void DRC::Reset( RESET_REASON aReason )
 
         m_pcb = m_pcbEditorFrame->GetBoard();
 
-        readRules();
+        loadRules();
     }
 }
 
@@ -354,35 +355,51 @@ int DRC::TestZoneToZoneOutlines()
 }
 
 
-void DRC::readRules()
+void DRC::loadRules()
 {
-    wxString rulesFilepath = m_pcbEditorFrame->Prj().AbsolutePath( "drc-rules" );
+    wxString   rulesFilepath = m_pcbEditorFrame->Prj().AbsolutePath( "drc-rules" );
+    wxFileName rulesFile( rulesFilepath );
 
-    BOARD_DESIGN_SETTINGS& bds = m_pcb->GetDesignSettings();
-    bds.m_DRCRuleSelectors.clear();
-    bds.m_DRCRules.clear();
-
-    FILE* fp = wxFopen( rulesFilepath, wxT( "rt" ) );
-
-    if( fp )
+    if( rulesFile.FileExists() )
     {
-        try
+        wxLongLong lastMod = rulesFile.GetModificationTime().GetValue();
+
+        if( lastMod > m_rulesFileLastMod )
         {
-            DRC_RULES_PARSER parser( m_pcb, fp, rulesFilepath );
-            parser.Parse( bds.m_DRCRuleSelectors, bds.m_DRCRules );
-        }
-        catch( PARSE_ERROR& pe )
-        {
-            DisplayError( m_drcDialog, pe.What() );
+            m_rulesFileLastMod = lastMod;
+            m_ruleSelectors.clear();
+            m_rules.clear();
+
+            FILE* fp = wxFopen( rulesFilepath, wxT( "rt" ) );
+
+            if( fp )
+            {
+                try
+                {
+                    DRC_RULES_PARSER parser( m_pcb, fp, rulesFilepath );
+                    parser.Parse( m_ruleSelectors, m_rules );
+                }
+                catch( PARSE_ERROR& pe )
+                {
+                    // Don't leave possibly malformed stuff around for us to trip over
+                    m_ruleSelectors.clear();
+                    m_rules.clear();
+
+                    DisplayError( m_drcDialog, pe.What() );
+                }
+            }
         }
     }
+
+    BOARD_DESIGN_SETTINGS& bds = m_pcb->GetDesignSettings();
+    bds.m_DRCRuleSelectors = m_ruleSelectors;
+    bds.m_DRCRules = m_rules;
 }
 
 
 void DRC::RunTests( wxTextCtrl* aMessages )
 {
-    // TODO: timestamp file and read only if newer
-    readRules();
+    loadRules();
 
     // be sure m_pcb is the current board, not a old one
     // ( the board can be reloaded )
diff --git a/pcbnew/drc/drc.h b/pcbnew/drc/drc.h
index 235e8dc0a6..52ee614fbf 100644
--- a/pcbnew/drc/drc.h
+++ b/pcbnew/drc/drc.h
@@ -159,15 +159,19 @@ private:
     bool     m_reportAllTrackErrors;    // Report all tracks errors (or only 4 first errors)
     bool     m_testFootprints;          // Test footprints against schematic
 
-    PCB_EDIT_FRAME*        m_pcbEditorFrame;   // The pcb frame editor which owns the board
-    BOARD*                 m_pcb;
-    SHAPE_POLY_SET         m_board_outlines;   // The board outline including cutouts
-    DIALOG_DRC*            m_drcDialog;
+    PCB_EDIT_FRAME*            m_pcbEditorFrame;   // The pcb frame editor which owns the board
+    BOARD*                     m_pcb;
+    SHAPE_POLY_SET             m_board_outlines;   // The board outline including cutouts
+    DIALOG_DRC*                m_drcDialog;
 
-    std::vector<DRC_ITEM*> m_unconnected;      // list of unconnected pads
-    std::vector<DRC_ITEM*> m_footprints;       // list of footprint warnings
-    bool                   m_drcRun;
-    bool                   m_footprintsTested;
+    std::vector<DRC_ITEM*>     m_unconnected;      // list of unconnected pads
+    std::vector<DRC_ITEM*>     m_footprints;       // list of footprint warnings
+    bool                       m_drcRun;
+    bool                       m_footprintsTested;
+
+    wxLongLong                 m_rulesFileLastMod;
+    std::vector<DRC_SELECTOR*> m_ruleSelectors;
+    std::vector<DRC_RULE*>     m_rules;
 
     ///> Sets up handlers for various events.
     void setTransitions() override;
@@ -177,7 +181,7 @@ private:
      */
     void updatePointers();
 
-    void readRules();
+    void loadRules();
 
     EDA_UNITS userUnits() const { return m_pcbEditorFrame->GetUserUnits(); }
 
diff --git a/pcbnew/drc/drc_rule_parser.cpp b/pcbnew/drc/drc_rule_parser.cpp
index 0a7a8d5ca5..7f722103a2 100644
--- a/pcbnew/drc/drc_rule_parser.cpp
+++ b/pcbnew/drc/drc_rule_parser.cpp
@@ -96,7 +96,17 @@ void DRC_RULES_PARSER::Parse( std::vector<DRC_SELECTOR*>& aSelectors,
         ruleMap[ rule->m_Name ] = rule;
 
     for( const std::pair<DRC_SELECTOR*, wxString>& entry : selectorRules )
-        entry.first->m_Rule = ruleMap[ entry.second ];
+    {
+        if( ruleMap.count( entry.second ) )
+        {
+            entry.first->m_Rule = ruleMap[ entry.second ];
+        }
+        else
+        {
+            wxString errText = wxString::Format( _( "Rule \"%s\" not found." ), entry.second );
+            THROW_PARSE_ERROR( errText, CurSource(), "", 0, 0 );
+        }
+    }
 }