Visual Odometry (VO) is a technique that allows knowing accurately the position of a robot over time, useful, for instance, for motion tracking, obstacle detection, avoidance, and autonomous navigation. To do these tasks requires the use of images captured by a monocular or stereo camera on a robot. From these images, it is needed to extract features to figure out how the camera is moving. This can be done in three different ways: feature matching, feature tracking, and calculating the Optical Flow. Once the key feature points are found is possible to do a 3D to 3D, 3D to 2D, or 2D to 2D motion estimation.