The use of cryptography, encryption and digital signatures is becoming more and more widespread in electronic communication. Nowadays they are an essential part of e-business and e-banking solutions, as well as other fields where the identity of the communicating parties has to be verified. Communication through secure (encrypted) channels is also becoming increasingly popular. This chapter offers a brief introduction into the fields of cryptography and the public key infrastructure (PKI), describing how they can be used for authentication and secure communication, and the PKI system developed for MS to support them.