The closed non-self-intersecting geodesics on the surface of a three-dimensional simplex are studied. It is proved that every geodesic on an arbitrary simplex can be realized on a regular simplex. This enables us to obtain a complete classification of all geodesics and describe their structure. Conditions for the existence of geodesics are obtained for an arbitrary simplex. It is proved that a simplex has infinitely many essentially different geodesics if and only if it is isohedral. Estimates for the number of geodesics are obtained for other Simplexes. Â© 2007 RAS(DoM) and LMS.