This course is the first course in the Understanding Modern Physics series, which covers an introduction to special relativity, general relativity and cosmology. We will find: (i) How space and time are relative to observers, and unified into a more fundamental construction of spacetime; (ii) Why the spacetime is not absolute, but rather curves in response to matter, and how gravity emerge as a result of such spacetime curvature; and (iii) What is the framework to understand the evolution of the whole universe