I defend a Humean theory of motivation on which desire motivates all action and drives all practical reasoning. First I lay out the significance for this view for metaethics, distinguishing it from a similar view defended by Michael Smith. Then I lay out four properties of desire. It motivates action, causes pleasant and unpleasant emotions, directs our attention, and is intensified by more vivid images of things we associate with its objects. I argue for the Humean theory on the grounds that it allows us to elegantly explain a variety of psychological phenomena involved in practical reasoning. These include the extent to which we succeed or fail in drawing conclusions in practical reasoning, the phenomenon of motivation connected to vivid representations which Tamar Gendler calls "alief". I also offer a Humean, externalist, and cognitivist account of moral motivation.