The McEliece cryptosystem is the first code-based public key cryptosystem proposed by Robert McEliece in 1978 a few years after the appearance of RSA. The original McEliece cryptosystem uses binary Goppa codes which are a subclass of Algebraic Geometric Codes and it is still unbroken under quantum attack. In this talk, we introduce basic facts about coding theory and discuss various code-based public key cryptosystems including our new cryptosystem McNie, which is a combination of the McEliece cryptosystem and the Niederreiter cryptosystem.