This is a upper division introduction to Special and General Relativity, with applications to theoretical and observational cosmology. This course is an APS minor elective, and is intended for science majors. We will delve into the reasons why relativity is important in studying cosmology, work through applications of SR and GR, and then jump from there to theoretical and observational cosmology. Because this is an astrophysics course, there will be strong emphasis on observational confirmations of Einstein’s theories, astrophysical applications of relativity including black holes, and finally the evidence for a Big Bang cosmology. We will follow this with discussion of the evolution of the universe, including synthesis of the elements, and the formation of structure. We will conclude (if time allows) with advanced topics on the inflationary period of the early universe and analyzing primordial fluctuations in the cosmic microwave background.