The focus in this book is on mobile robots that move on a surface. When the robot moves for a period of time its new position can be determined by odometry: integrating the velocity of the robot over the period of its motion to obtain distance or integrating the acceleration to get velocity and integrating again to obtain distance. If the robot changes its heading as it moves, trigonometry is needed to compute the new position. Odometry is subject to errors caused by uncertainty in the components of the robot and unevenness of the surface. Wheel encoders enable more accurate odometry. Inertial navigation systems perform odometry based on accurate measurement of linear and angular acceleration. The degrees of freedom (DOF) of a system is the number dimensions through which it can move. The number of actuators of the robot can be more or less than the DOF. The DOF can be different from the degrees of mobility (DOM), which are the DOF that can be directly accessed. The concept of holonomic motion relates the DOF and the DOM.