diff --git a/CMakeLists.txt b/CMakeLists.txt
index 08a8af468e..efc9480f0f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -28,7 +28,7 @@ endif()
 
 LIST(APPEND CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/cmake_modules)
 
-find_package(OpenCV 3.0 QUIET)
+find_package(OpenCV 4.0 QUIET)
 if(NOT OpenCV_FOUND)
    find_package(OpenCV 2.4.3 QUIET)
    if(NOT OpenCV_FOUND)
diff --git a/Examples/Monocular/mono_euroc.cc b/Examples/Monocular/mono_euroc.cc
index 4bcb90f170..045cd34dcd 100644
--- a/Examples/Monocular/mono_euroc.cc
+++ b/Examples/Monocular/mono_euroc.cc
@@ -70,7 +70,8 @@ int main(int argc, char **argv)
     for(int ni=0; ni<nImages; ni++)
     {
         // Read image from file
-        im = cv::imread(vstrImageFilenames[ni],CV_LOAD_IMAGE_UNCHANGED);
+        //im = cv::imread(vstrImageFilenames[ni],CV_LOAD_IMAGE_UNCHANGED);
+        im = cv::imread(vstrImageFilenames[ni],cv::IMREAD_UNCHANGED);
         double tframe = vTimestamps[ni];
 
         if(im.empty())
diff --git a/Examples/Monocular/mono_kitti.cc b/Examples/Monocular/mono_kitti.cc
index f2f7b3ebfd..daa27362fc 100644
--- a/Examples/Monocular/mono_kitti.cc
+++ b/Examples/Monocular/mono_kitti.cc
@@ -65,7 +65,7 @@ int main(int argc, char **argv)
     for(int ni=0; ni<nImages; ni++)
     {
         // Read image from file
-        im = cv::imread(vstrImageFilenames[ni],CV_LOAD_IMAGE_UNCHANGED);
+        im = cv::imread(vstrImageFilenames[ni],cv::IMREAD_UNCHANGED);
         double tframe = vTimestamps[ni];
 
         if(im.empty())
diff --git a/Examples/Monocular/mono_tum.cc b/Examples/Monocular/mono_tum.cc
index 09a2afc4b7..3c9000628b 100644
--- a/Examples/Monocular/mono_tum.cc
+++ b/Examples/Monocular/mono_tum.cc
@@ -65,7 +65,7 @@ int main(int argc, char **argv)
     for(int ni=0; ni<nImages; ni++)
     {
         // Read image from file
-        im = cv::imread(string(argv[3])+"/"+vstrImageFilenames[ni],CV_LOAD_IMAGE_UNCHANGED);
+        im = cv::imread(string(argv[3])+"/"+vstrImageFilenames[ni],cv::IMREAD_UNCHANGED);
         double tframe = vTimestamps[ni];
 
         if(im.empty())
diff --git a/Examples/RGB-D/rgbd_tum.cc b/Examples/RGB-D/rgbd_tum.cc
index 001199d1aa..b2a3dd0e4b 100644
--- a/Examples/RGB-D/rgbd_tum.cc
+++ b/Examples/RGB-D/rgbd_tum.cc
@@ -77,8 +77,8 @@ int main(int argc, char **argv)
     for(int ni=0; ni<nImages; ni++)
     {
         // Read image and depthmap from file
-        imRGB = cv::imread(string(argv[3])+"/"+vstrImageFilenamesRGB[ni],CV_LOAD_IMAGE_UNCHANGED);
-        imD = cv::imread(string(argv[3])+"/"+vstrImageFilenamesD[ni],CV_LOAD_IMAGE_UNCHANGED);
+        imRGB = cv::imread(string(argv[3])+"/"+vstrImageFilenamesRGB[ni], cv::IMREAD_UNCHANGED );
+        imD = cv::imread(string(argv[3])+"/"+vstrImageFilenamesD[ni],cv::IMREAD_UNCHANGED );
         double tframe = vTimestamps[ni];
 
         if(imRGB.empty())
diff --git a/Examples/Stereo/stereo_euroc.cc b/Examples/Stereo/stereo_euroc.cc
index 6bc09c50be..99fb25280c 100644
--- a/Examples/Stereo/stereo_euroc.cc
+++ b/Examples/Stereo/stereo_euroc.cc
@@ -116,8 +116,8 @@ int main(int argc, char **argv)
     for(int ni=0; ni<nImages; ni++)
     {
         // Read left and right images from file
-        imLeft = cv::imread(vstrImageLeft[ni],CV_LOAD_IMAGE_UNCHANGED);
-        imRight = cv::imread(vstrImageRight[ni],CV_LOAD_IMAGE_UNCHANGED);
+        imLeft = cv::imread(vstrImageLeft[ni],cv::IMREAD_UNCHANGED);
+        imRight = cv::imread(vstrImageRight[ni],cv::IMREAD_UNCHANGED);
 
         if(imLeft.empty())
         {
diff --git a/Examples/Stereo/stereo_kitti.cc b/Examples/Stereo/stereo_kitti.cc
index cb8bc4096e..f6c5869761 100644
--- a/Examples/Stereo/stereo_kitti.cc
+++ b/Examples/Stereo/stereo_kitti.cc
@@ -66,8 +66,8 @@ int main(int argc, char **argv)
     for(int ni=0; ni<nImages; ni++)
     {
         // Read left and right images from file
-        imLeft = cv::imread(vstrImageLeft[ni],CV_LOAD_IMAGE_UNCHANGED);
-        imRight = cv::imread(vstrImageRight[ni],CV_LOAD_IMAGE_UNCHANGED);
+        imLeft = cv::imread(vstrImageLeft[ni],cv::IMREAD_UNCHANGED);
+        imRight = cv::imread(vstrImageRight[ni],cv::IMREAD_UNCHANGED);
         double tframe = vTimestamps[ni];
 
         if(imLeft.empty())
diff --git a/Thirdparty/DBoW2/CMakeLists.txt b/Thirdparty/DBoW2/CMakeLists.txt
index 0eb512600a..c5a1a1a2d1 100644
--- a/Thirdparty/DBoW2/CMakeLists.txt
+++ b/Thirdparty/DBoW2/CMakeLists.txt
@@ -24,7 +24,7 @@ set(SRCS_DUTILS
   DUtils/Random.cpp
   DUtils/Timestamp.cpp)
 
-find_package(OpenCV 3.0 QUIET)
+find_package(OpenCV 4.0 QUIET)
 if(NOT OpenCV_FOUND)
    find_package(OpenCV 2.4.3 QUIET)
    if(NOT OpenCV_FOUND)
diff --git a/include/LocalMapping.h b/include/LocalMapping.h
index 1361a5757a..7147a0a83c 100644
--- a/include/LocalMapping.h
+++ b/include/LocalMapping.h
@@ -26,6 +26,7 @@
 #include "LoopClosing.h"
 #include "Tracking.h"
 #include "KeyFrameDatabase.h"
+#include <unistd.h>
 
 #include <mutex>
 
diff --git a/include/LoopClosing.h b/include/LoopClosing.h
index 7eb0416b15..d74bdd03c9 100644
--- a/include/LoopClosing.h
+++ b/include/LoopClosing.h
@@ -46,8 +46,11 @@ class LoopClosing
 public:
 
     typedef pair<set<KeyFrame*>,int> ConsistentGroup;    
+    // typedef map<KeyFrame*,g2o::Sim3,std::less<KeyFrame*>,
+    //     Eigen::aligned_allocator<std::pair<const KeyFrame*, g2o::Sim3> > > KeyFrameAndPose;
+
     typedef map<KeyFrame*,g2o::Sim3,std::less<KeyFrame*>,
-        Eigen::aligned_allocator<std::pair<const KeyFrame*, g2o::Sim3> > > KeyFrameAndPose;
+        Eigen::aligned_allocator<std::pair<KeyFrame *const, g2o::Sim3> > > KeyFrameAndPose;
 
 public:
 
diff --git a/include/ORBextractor.h b/include/ORBextractor.h
index 66e8e7a547..b6bc110396 100644
--- a/include/ORBextractor.h
+++ b/include/ORBextractor.h
@@ -23,7 +23,8 @@
 
 #include <vector>
 #include <list>
-#include <opencv/cv.h>
+//#include <opencv/cv.h> //opencv3
+#include <opencv2/imgproc/types_c.h>
 
 
 namespace ORB_SLAM2