In video sequences, motion is a key source of information. Motion arises due to moving objects in the 3D scene, as well as camera motion. Apparent motion, also known as optical flow, captures the resulting spatio-temporal variations of pixel intensities in successive images of a sequence. The purpose of motion estimation techniques is to recover this information by analyzing the image content. Efficient and accurate motion estimation is an essential component in many image sequence analysis, computer vision and video communication applications. Motion estimation is a vast topic. The purpose of this chapter is to give a broad overview of motion estimation techniques with a special emphasis on video compression requirements.