This is an introductory course on elliptic curves. Elliptic curves play important roles in mathematics as well as it found applications in cryptography. Our emphasis will be on the arithmetic aspect of the theory (i.e. over finite fields and number fields). One of our goals is the proof of the Mordell-Weil theorem on the finite generatedness of the group of rational points of an elliptic curve over a number field. As for prerequisites, we assume basic algebra and (general) topology, but we do not assume a prior knowledge of algebraic geometry. Some knowledge of algebraic number theory may be helpful but not essential.