diff --git a/Libs/Core/ctkUtils.cpp b/Libs/Core/ctkUtils.cpp
index d22201f019..e055ff7ade 100644
--- a/Libs/Core/ctkUtils.cpp
+++ b/Libs/Core/ctkUtils.cpp
@@ -21,7 +21,11 @@
 // Qt includes
 #include <QDebug>
 #include <QDir>
+#if (QT_VERSION >= QT_VERSION_CHECK(6, 0, 0))
+#include <QRegularExpression>
+#else
 #include <QRegExp>
+#endif
 #include <QString>
 #include <QStringList>
 
@@ -109,17 +113,30 @@ const char *ctkValidWildCard =
 QStringList ctk::nameFilterToExtensions(const QString& nameFilter)
 {
   QRegExp regexp(QString::fromLatin1(ctkNameFilterRegExp));
+#if (QT_VERSION >= QT_VERSION_CHECK(6, 0, 0))
+  int i = nameFilter.indexOf(regexp);
+#else
   int i = regexp.indexIn(nameFilter);
+#endif
   if (i < 0)
     {
     QRegExp isWildCard(QString::fromLatin1(ctkValidWildCard));
+#if (QT_VERSION >= QT_VERSION_CHECK(6, 0, 0))
+    if (nameFilter.indexOf(isWildCard) >= 0)
+#else
     if (isWildCard.indexIn(nameFilter) >= 0)
+#endif
       {
       return QStringList(nameFilter);
       }
     return QStringList();
     }
+#if (QT_VERSION >= QT_VERSION_CHECK(6, 0, 0))
+  QStringList captured = regexp.namedCaptureGroups();
+  QString f = captured.size() >= 3 ? captured[2] : "";
+#else
   QString f = regexp.cap(2);
+#endif
   #if (QT_VERSION >= QT_VERSION_CHECK(5, 14, 0))
   return f.split(QLatin1Char(' '), Qt::SkipEmptyParts);
   #else
@@ -143,12 +160,22 @@ QString ctk::extensionToRegExp(const QString& extension)
 {
   // typically *.jpg
   QRegExp extensionExtractor("\\*\\.(\\w+)");
+#if (QT_VERSION >= QT_VERSION_CHECK(6, 0, 0))
+  int pos = extension.indexOf(extensionExtractor);
+#else
   int pos = extensionExtractor.indexIn(extension);
+#endif
   if (pos < 0)
     {
     return QString();
     }
-  return ".*\\." + extensionExtractor.cap(1) + "?$";
+#if (QT_VERSION >= QT_VERSION_CHECK(6, 0, 0))
+  QStringList captured = extensionExtractor.namedCaptureGroups();
+  QString cap = captured.size() >= 2 ? captured[1] : "";
+#else
+  QString cap = extensionExtractor.cap(1);
+#endif
+  return ".*\\." + cap + "?$";
 }
 
 //-----------------------------------------------------------------------------
diff --git a/Libs/Core/ctkUtils.h b/Libs/Core/ctkUtils.h
index 5ddd7c774f..605f9e3e1d 100644
--- a/Libs/Core/ctkUtils.h
+++ b/Libs/Core/ctkUtils.h
@@ -28,6 +28,10 @@
 #include <QModelIndex>
 #include <QStringList>
 
+#if (QT_VERSION >= QT_VERSION_CHECK(6, 0, 0))
+#define QRegExp QRegularExpression
+#endif
+
 // STD includes
 #include <vector>
 
diff --git a/Libs/Widgets/ctkBasePopupWidget.cpp b/Libs/Widgets/ctkBasePopupWidget.cpp
index 8348161c7e..0962eea117 100644
--- a/Libs/Widgets/ctkBasePopupWidget.cpp
+++ b/Libs/Widgets/ctkBasePopupWidget.cpp
@@ -21,7 +21,9 @@
 // Qt includes
 #include <QApplication>
 #include <QDebug>
+#if (QT_VERSION < QT_VERSION_CHECK(6, 0, 0))
 #include <QDesktopWidget>
+#endif
 #include <QDir>
 #include <QEvent>
 #include <QLabel>
diff --git a/Libs/Widgets/ctkDoubleSpinBox.cpp b/Libs/Widgets/ctkDoubleSpinBox.cpp
index c263823199..96fdb72518 100644
--- a/Libs/Widgets/ctkDoubleSpinBox.cpp
+++ b/Libs/Widgets/ctkDoubleSpinBox.cpp
@@ -398,13 +398,24 @@ ::validateAndInterpret(QString &input, int &pos,
   // could be because of group separators:
   if (!ok && state == QValidator::Acceptable)
     {
-    if (q->locale().groupSeparator().isPrint())
+#if (QT_VERSION >= QT_VERSION_CHECK(6, 0, 0))
+    QChar groupSeparator;
+    if ( q->locale().groupSeparator().size() == 1 )
+      {
+        groupSeparator = q->locale().groupSeparator()[0];
+      }
+    // else: group separator does not necessarily fit into a QChar (https://bugreports.qt.io/browse/QTBUG-69324) 
+    // but CTK only support group separators if they fit into a QChar
+#else
+    QChar groupSeparator = q->locale().groupSeparator();
+#endif
+    if (groupSeparator.isPrint())
       {
       int start = (dec == -1 ? text.size() : dec)- 1;
       int lastGroupSeparator = start;
       for (int digit = start; digit >= 0; --digit)
         {
-        if (text.at(digit) == q->locale().groupSeparator())
+        if (text.at(digit) == groupSeparator)
           {
           if (digit != lastGroupSeparator - 3)
             {
@@ -1101,8 +1112,11 @@ QSize ctkDoubleSpinBox::sizeHint() const
 
   opt.rect = this->rect();
   d->CachedSizeHint = this->style()->sizeFromContents(
-    QStyle::CT_SpinBox, &opt, newSizeHint, this)
-    .expandedTo(QApplication::globalStrut());
+    QStyle::CT_SpinBox, &opt, newSizeHint, this
+#if (QT_VERSION < QT_VERSION_CHECK(6, 0, 0))
+    ).expandedTo(QApplication::globalStrut()
+#endif
+    );
   return d->CachedSizeHint;
 }
 
@@ -1157,8 +1171,11 @@ QSize ctkDoubleSpinBox::minimumSizeHint() const
 
   opt.rect = this->rect();
   d->CachedMinimumSizeHint = this->style()->sizeFromContents(
-    QStyle::CT_SpinBox, &opt, newSizeHint, this)
-    .expandedTo(QApplication::globalStrut());
+    QStyle::CT_SpinBox, &opt, newSizeHint, this
+#if (QT_VERSION < QT_VERSION_CHECK(6, 0, 0))
+    ).expandedTo(QApplication::globalStrut()
+#endif
+    );
   return d->CachedMinimumSizeHint;
 }
 
diff --git a/Libs/Widgets/ctkPopupWidget.cpp b/Libs/Widgets/ctkPopupWidget.cpp
index 1015547f12..b01662549f 100644
--- a/Libs/Widgets/ctkPopupWidget.cpp
+++ b/Libs/Widgets/ctkPopupWidget.cpp
@@ -20,7 +20,9 @@
 
 // Qt includes
 #include <QApplication>
+#if (QT_VERSION < QT_VERSION_CHECK(6, 0, 0))
 #include <QDesktopWidget>
+#endif
 #include <QDialog>
 #include <QDir>
 #include <QEvent>
@@ -416,7 +418,11 @@ void ctkPopupWidget::leaveEvent(QEvent* event)
 }
 
 // --------------------------------------------------------------------------
+#if (QT_VERSION >= QT_VERSION_CHECK(6, 0, 0))
+void ctkPopupWidget::enterEvent(QEnterEvent* event)
+#else
 void ctkPopupWidget::enterEvent(QEvent* event)
+#endif
 {
   Q_D(ctkPopupWidget);
   QTimer::singleShot(d->ShowDelay, this, SLOT(updatePopup()));
diff --git a/Libs/Widgets/ctkPopupWidget.h b/Libs/Widgets/ctkPopupWidget.h
index 99675c544e..7ab8f5b7d9 100644
--- a/Libs/Widgets/ctkPopupWidget.h
+++ b/Libs/Widgets/ctkPopupWidget.h
@@ -113,7 +113,11 @@ public Q_SLOTS:
 
 protected:
   virtual void leaveEvent(QEvent* event);
+#if (QT_VERSION >= QT_VERSION_CHECK(6, 0, 0))
+  virtual void enterEvent(QEnterEvent* event);
+#else
   virtual void enterEvent(QEvent* event);
+#endif
   virtual bool eventFilter(QObject* obj, QEvent* event);
 
   /// Widget the popup is attached to. It opens right under \a baseWidget
diff --git a/Libs/Widgets/ctkRangeSlider.cpp b/Libs/Widgets/ctkRangeSlider.cpp
index a20a99b242..48fd0b9038 100644
--- a/Libs/Widgets/ctkRangeSlider.cpp
+++ b/Libs/Widgets/ctkRangeSlider.cpp
@@ -111,7 +111,7 @@ ctkRangeSliderPrivate::ctkRangeSliderPrivate(ctkRangeSlider& object)
   this->m_SubclassClickOffset = 0;
   this->m_SubclassPosition = 0;
   this->m_SubclassWidth = 0.0;
-  this->m_SelectedHandles = 0;
+  this->m_SelectedHandles = ctkRangeSliderPrivate::Handles(); // initialize QFlags with no flags 
   this->m_SymmetricMoves = false;
 }
 
@@ -773,7 +773,7 @@ void ctkRangeSlider::mouseReleaseEvent(QMouseEvent* mouseEvent)
   this->QSlider::mouseReleaseEvent(mouseEvent);
 
   setSliderDown(false);
-  d->m_SelectedHandles = 0;
+  d->m_SelectedHandles = ctkRangeSliderPrivate::Handles(); // initialize QFlags with no flags set
 
   this->update();
 }