OpenCV headers


This may be a simple question but it eludes me. I noticed that to use OpenCV (or other external libraries) Jules recommends including them before any of the Juce stuff. This seems to work fine if I include them first in a .cpp file. However in a .h file it doesn't matter where I include the headers, the project won't compile. 

Is this normal?

If I can't include the OpenCV headers in my class header file, how can I declare CV type member objects like cv::Mat?