Analyzing images for certain objects or matching two or more images are frequent tasks that medical imaging systems have to be able to perform with respect to their dedicated purpose. Both tasks can be solved by scanning the images for specific features that characterize the targeted image contents. Often one requires an overlay of images from two different modalities to merge the information, or one wants to analyze the bloodflow in an organ. For all these tasks, we introduce important feature descriptors, explain the underlying principles, and present examples and applications. At the end of this module you will have learned about the structure tensor and Harris corner detector, the vesselness filter, the concept of scale space, SIFT, and about the challenges involved with feature matching.