diff --git a/common/lib_table_base.cpp b/common/lib_table_base.cpp
index 525d9ddfd6..8b72713027 100644
--- a/common/lib_table_base.cpp
+++ b/common/lib_table_base.cpp
@@ -185,6 +185,8 @@ LIB_TABLE_ROW* LIB_TABLE::findRow( const wxString& aNickName, bool aCheckIfEnabl
     LIB_TABLE_ROW* row = nullptr;
     LIB_TABLE* cur = (LIB_TABLE*) this;
 
+    std::lock_guard<std::recursive_mutex> lock( m_nickIndexMutex );
+
     do
     {
         cur->ensureIndex();
@@ -301,6 +303,8 @@ std::vector<wxString> LIB_TABLE::GetLogicalLibs()
 
 bool LIB_TABLE::InsertRow( LIB_TABLE_ROW* aRow, bool doReplace )
 {
+    std::lock_guard<std::recursive_mutex> lock( m_nickIndexMutex );
+
     ensureIndex();
 
     INDEX_CITER it = nickIndex.find( aRow->GetNickName() );
diff --git a/common/template_fieldnames.cpp b/common/template_fieldnames.cpp
index f114f37f7c..3375638ee3 100644
--- a/common/template_fieldnames.cpp
+++ b/common/template_fieldnames.cpp
@@ -22,6 +22,8 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
  */
 
+#include <mutex>
+
 #include <macros.h>
 #include <template_fieldnames.h>
 #include <pgm_base.h>
@@ -33,6 +35,8 @@ using namespace TFIELD_T;
 #define FTPCANONICAL "Footprint"
 #define DSHCANONICAL "Datasheet"
 
+static std::mutex s_defaultFieldMutex;
+
 const wxString TEMPLATE_FIELDNAME::GetDefaultFieldName( int aFieldNdx, bool aTranslate )
 {
     static void* locale = nullptr;
@@ -53,6 +57,10 @@ const wxString TEMPLATE_FIELDNAME::GetDefaultFieldName( int aFieldNdx, bool aTra
         }
     }
 
+    // Mutex protection is needed so that multiple loader threads don't write to the static variables
+    // at once
+    std::lock_guard<std::mutex> lock( s_defaultFieldMutex );
+
     // Fetching translations can take a surprising amount of time when loading libraries,
     // so only do it when necessary.
     if( Pgm().GetLocale() != locale )
diff --git a/eeschema/class_library.cpp b/eeschema/class_library.cpp
index 234281d8f5..1d00a366c2 100644
--- a/eeschema/class_library.cpp
+++ b/eeschema/class_library.cpp
@@ -56,7 +56,7 @@
 PART_LIB::PART_LIB( SCH_LIB_TYPE aType, const wxString& aFileName, SCH_IO_MGR::SCH_FILE_T aPluginType ) :
     // start @ != 0 so each additional library added
     // is immediately detectable, zero would not be.
-    m_mod_hash( PART_LIBS::s_modify_generation ),
+    m_mod_hash( PART_LIBS::GetModifyGeneration() ),
     m_pluginType( aPluginType )
 {
     type = aType;
@@ -413,7 +413,8 @@ void PART_LIBS::FindLibraryNearEntries( std::vector<LIB_PART*>& aCandidates,
 }
 
 
-int PART_LIBS::s_modify_generation = 1;     // starts at 1 and goes up
+int        PART_LIBS::s_modify_generation = 1;     // starts at 1 and goes up
+std::mutex PART_LIBS::s_generationMutex;
 
 
 int PART_LIBS::GetModifyHash()
@@ -428,7 +429,7 @@ int PART_LIBS::GetModifyHash()
     // Rebuilding the cache (m_cache) does not change the GetModHash() value,
     // but changes PART_LIBS::s_modify_generation.
     // Take this change in account:
-    hash += PART_LIBS::s_modify_generation;
+    hash += PART_LIBS::GetModifyGeneration();
 
     return hash;
 }
diff --git a/eeschema/class_library.h b/eeschema/class_library.h
index cc6f27a027..40e2e65631 100644
--- a/eeschema/class_library.h
+++ b/eeschema/class_library.h
@@ -32,6 +32,7 @@
 #define CLASS_LIBRARY_H
 
 #include <map>
+#include <mutex>
 #include <boost/ptr_container/ptr_vector.hpp>
 #include <wx/filename.h>
 
@@ -197,11 +198,24 @@ class PART_LIBS : public PART_LIBS_BASE, public PROJECT::_ELEM
 public:
     KICAD_T Type() override { return PART_LIBS_T; }
 
-    static int s_modify_generation;         ///< helper for GetModifyHash()
+    static int        s_modify_generation;         ///< helper for GetModifyHash()
+    static std::mutex s_generationMutex;
 
     PART_LIBS()
     {
-        ++s_modify_generation;
+        IncrementModifyGeneration();
+    }
+
+    static void IncrementModifyGeneration()
+    {
+        std::lock_guard<std::mutex> mut( PART_LIBS::s_generationMutex );
+        ++PART_LIBS::s_modify_generation;
+    }
+
+    static int GetModifyGeneration()
+    {
+        std::lock_guard<std::mutex> mut( PART_LIBS::s_generationMutex );
+        return PART_LIBS::s_modify_generation;
     }
 
     /// Return the modification hash for all libraries.  The value returned
diff --git a/eeschema/eeschema_config.cpp b/eeschema/eeschema_config.cpp
index b4d90b883d..4378bba585 100644
--- a/eeschema/eeschema_config.cpp
+++ b/eeschema/eeschema_config.cpp
@@ -21,6 +21,8 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
  */
 
+#include <mutex>
+
 #include <class_library.h>
 #include <confirm.h>
 #include <dialogs/panel_eeschema_color_settings.h>
@@ -224,8 +226,14 @@ void SYMBOL_EDIT_FRAME::InstallPreferences( PAGED_DIALOG* aParent,
 }
 
 
+static std::mutex s_symbolTableMutex;
+
+
 SYMBOL_LIB_TABLE* PROJECT::SchSymbolLibTable()
 {
+    wxLogDebug( "Getting symbol lib table" );
+    std::lock_guard<std::mutex> lock( s_symbolTableMutex );
+
     // This is a lazy loading function, it loads the project specific table when
     // that table is asked for, not before.
     SYMBOL_LIB_TABLE* tbl = (SYMBOL_LIB_TABLE*) GetElem( ELEM_SYMBOL_LIB_TABLE );
diff --git a/eeschema/sch_plugins/kicad/sch_sexpr_plugin.cpp b/eeschema/sch_plugins/kicad/sch_sexpr_plugin.cpp
index f45133c0c8..f71fc517df 100644
--- a/eeschema/sch_plugins/kicad/sch_sexpr_plugin.cpp
+++ b/eeschema/sch_plugins/kicad/sch_sexpr_plugin.cpp
@@ -777,7 +777,7 @@ void SCH_SEXPR_PLUGIN::Format( SCH_SHEET* aSheet )
 }
 
 
-void SCH_SEXPR_PLUGIN::Format( EE_SELECTION* aSelection, SCH_SHEET_PATH* aSheetPath, 
+void SCH_SEXPR_PLUGIN::Format( EE_SELECTION* aSelection, SCH_SHEET_PATH* aSheetPath,
                                OUTPUTFORMATTER* aFormatter )
 {
     wxCHECK( aSelection && aFormatter, /* void */ );
@@ -2094,7 +2094,7 @@ void SCH_SEXPR_PLUGIN::cacheLib( const wxString& aLibraryFileName, const PROPERT
         // Because m_cache is rebuilt, increment PART_LIBS::s_modify_generation
         // to modify the hash value that indicate component to symbol links
         // must be updated.
-        PART_LIBS::s_modify_generation++;
+        PART_LIBS::IncrementModifyGeneration();
 
         if( !isBuffering( aProperties ) )
             m_cache->Load();
diff --git a/eeschema/sch_plugins/legacy/sch_legacy_plugin.cpp b/eeschema/sch_plugins/legacy/sch_legacy_plugin.cpp
index cc04ba0ae6..6e61e9d580 100644
--- a/eeschema/sch_plugins/legacy/sch_legacy_plugin.cpp
+++ b/eeschema/sch_plugins/legacy/sch_legacy_plugin.cpp
@@ -23,6 +23,7 @@
 #include <algorithm>
 #include <boost/algorithm/string/join.hpp>
 #include <cctype>
+#include <mutex>
 #include <set>
 
 #include <wx/mstream.h>
@@ -476,8 +477,7 @@ static void parseQuotedString( wxString& aString, LINE_READER& aReader,
  */
 class SCH_LEGACY_PLUGIN_CACHE
 {
-    static int      m_modHash;      // Keep track of the modification status of the library.
-
+    static int      s_modHash;      // Keep track of the modification status of the library.
     wxString        m_fileName;     // Absolute path and file name.
     wxFileName      m_libFileName;  // Absolute path and file name is required here.
     wxDateTime      m_fileModTime;
@@ -523,11 +523,23 @@ class SCH_LEGACY_PLUGIN_CACHE
 
     friend SCH_LEGACY_PLUGIN;
 
+    static std::mutex s_modHashMutex;
+
 public:
     SCH_LEGACY_PLUGIN_CACHE( const wxString& aLibraryPath );
     ~SCH_LEGACY_PLUGIN_CACHE();
 
-    int GetModifyHash() const { return m_modHash; }
+    static void IncrementModifyHash()
+    {
+        std::lock_guard<std::mutex> mut( SCH_LEGACY_PLUGIN_CACHE::s_modHashMutex );
+        SCH_LEGACY_PLUGIN_CACHE::s_modHash++;
+    }
+
+    static int GetModifyHash()
+    {
+        std::lock_guard<std::mutex> mut( SCH_LEGACY_PLUGIN_CACHE::s_modHashMutex );
+        return SCH_LEGACY_PLUGIN_CACHE::s_modHash;
+    }
 
     // Most all functions in this class throw IO_ERROR exceptions.  There are no
     // error codes nor user interface calls from here, nor in any SCH_PLUGIN objects.
@@ -2395,7 +2407,8 @@ void SCH_LEGACY_PLUGIN::saveBusAlias( std::shared_ptr<BUS_ALIAS> aAlias )
 }
 
 
-int SCH_LEGACY_PLUGIN_CACHE::m_modHash = 1;     // starts at 1 and goes up
+int SCH_LEGACY_PLUGIN_CACHE::s_modHash = 1;     // starts at 1 and goes up
+std::mutex SCH_LEGACY_PLUGIN_CACHE::s_modHashMutex;
 
 
 SCH_LEGACY_PLUGIN_CACHE::SCH_LEGACY_PLUGIN_CACHE( const wxString& aFullPathAndFileName ) :
@@ -2533,7 +2546,7 @@ LIB_PART* SCH_LEGACY_PLUGIN_CACHE::removeSymbol( LIB_PART* aPart )
     m_symbols.erase( it );
     delete aPart;
     m_isModified = true;
-    ++m_modHash;
+    SCH_LEGACY_PLUGIN_CACHE::IncrementModifyHash();
     return firstChild;
 }
 
@@ -2551,7 +2564,7 @@ void SCH_LEGACY_PLUGIN_CACHE::AddSymbol( const LIB_PART* aPart )
 
     m_symbols[ name ] = const_cast< LIB_PART* >( aPart );
     m_isModified = true;
-    ++m_modHash;
+    SCH_LEGACY_PLUGIN_CACHE::IncrementModifyHash();
 }
 
 
@@ -2631,7 +2644,7 @@ void SCH_LEGACY_PLUGIN_CACHE::Load()
         }
     }
 
-    ++m_modHash;
+    SCH_LEGACY_PLUGIN_CACHE::IncrementModifyHash();
 
     // Remember the file modification time of library file when the
     // cache snapshot was made, so that in a networked environment we will
@@ -4199,7 +4212,7 @@ void SCH_LEGACY_PLUGIN_CACHE::DeleteSymbol( const wxString& aSymbolName )
         delete part;
     }
 
-    ++m_modHash;
+    SCH_LEGACY_PLUGIN_CACHE::IncrementModifyHash();
     m_isModified = true;
 }
 
@@ -4215,7 +4228,7 @@ void SCH_LEGACY_PLUGIN::cacheLib( const wxString& aLibraryFileName, const PROPER
         // Because m_cache is rebuilt, increment PART_LIBS::s_modify_generation
         // to modify the hash value that indicate component to symbol links
         // must be updated.
-        PART_LIBS::s_modify_generation++;
+        PART_LIBS::IncrementModifyGeneration();
 
         if( !isBuffering( aProperties ) )
             m_cache->Load();
@@ -4243,7 +4256,7 @@ bool SCH_LEGACY_PLUGIN::isBuffering( const PROPERTIES* aProperties )
 int SCH_LEGACY_PLUGIN::GetModifyHash() const
 {
     if( m_cache )
-        return m_cache->GetModifyHash();
+        return SCH_LEGACY_PLUGIN_CACHE::GetModifyHash();
 
     // If the cache hasn't been loaded, it hasn't been modified.
     return 0;
diff --git a/include/lib_table_base.h b/include/lib_table_base.h
index 382087f9b8..fbd4f9af2b 100644
--- a/include/lib_table_base.h
+++ b/include/lib_table_base.h
@@ -30,6 +30,7 @@
 #include <boost/noncopyable.hpp>
 #include <boost/ptr_container/ptr_vector.hpp>
 #include <memory>
+#include <mutex>
 #include <project.h>
 #include <properties.h>
 #include <richio.h>
@@ -318,6 +319,8 @@ public:
     /// Delete all rows.
     void Clear()
     {
+        std::lock_guard<std::recursive_mutex> lock( m_nickIndexMutex );
+
         rows.clear();
         nickIndex.clear();
     }
@@ -501,6 +504,8 @@ protected:
 
     void reindex()
     {
+        std::lock_guard<std::recursive_mutex> lock( m_nickIndexMutex );
+
         nickIndex.clear();
 
         for( LIB_TABLE_ROWS_ITER it = rows.begin(); it != rows.end(); ++it )
@@ -509,6 +514,8 @@ protected:
 
     void ensureIndex()
     {
+        std::lock_guard<std::recursive_mutex> lock( m_nickIndexMutex );
+
         // The dialog lib table editor may not maintain the nickIndex.
         // Lazy indexing may be required.  To handle lazy indexing, we must enforce
         // that "nickIndex" is either empty or accurate, but never inaccurate.
@@ -533,6 +540,9 @@ protected:
     INDEX nickIndex;
 
     LIB_TABLE* fallBack;
+
+    /// Mutex to protect access to the nickIndex variable
+    mutable std::recursive_mutex m_nickIndexMutex;
 };
 
 #endif  // _LIB_TABLE_BASE_H_