Galilean Relativity and its incompatibility with electrodynamics. Einstein's reformulation of The Relativity Principle and its consequences such as frame-dependent simultaneity of events, dilation of time intervals and contraction of lengths, Lorentz transformations. The interchangeability of mass and energy. The equivalence of accelerating frames and gravitational fields, aspects of General relativity and Cosmology. The treatment emphasizes physical ideas rather than mathematics, in particular, no knowledge of Calculus is needed. A popular-level book from 1916 by Einstein will be the main text.