All else being equal, evolution is going to drive animals to require the least food to move a unit distance. What is the best way to do that? Some efficiencies can be 'hard-wired' into the body - the relatively unchanging morphology of the animal. But flexibility is also needed - given the task at hand, state of the body, or state of the external environment, it may be best to dynamically choose an appropriate mode of locomotion. For example, slow walking may be great for searching and foraging, but it won't catch fast moving prey. Similarly, maximum speed gallops may be great for escaping danger, but they preclude eating along the way. This primer summarizes what we know about the determinants of locomotor costs and the strategies animals use to minimize cost. It summarizes key findings across levels of organization, from individual muscles to interactions with other organisms and the environment. At the mid-level of organization we highlight gaits, a particularly interesting topic of inquiry with a rich history. We are in an exciting time for the science of movement because we have more, better tools than ever before for observing and manipulating systems, from the molecular level to herds of animals on the Savannah. Even more importantly, there are so many open, exciting questions to ask.