Cryptographic standards serve two important goals: making different implementations interoperable and avoiding various known pitfalls in commonly used schemes. This chapter discusses Public-Key Cryptography Standards (PKCS) which have significant impact on the use of public key cryptography in practice. PKCS standards are a set of standards, called PKCS #1 through #15. These standards cover RSA encryption, RSA signature, password-based encryption, cryptographic message syntax, private-key information syntax, selected object classes and attribute types, certification request syntax, cryptographic token interface, personal information exchange syntax, and cryptographic token information syntax. The PKCS standards are published by RSA Laboratories. Though RSA Laboratories solicits public opinions and advice for PKCS standards, RSA Laboratories retain sole decision-making authority on all aspects of PKCS standards. PKCS has been the basis for many other standards such as S/MIME.