Hash based signatures (HBS) are a type of post-quantum digital signature that use hash functions as their central building block. They are efficient, flexible, and can be used in a variety of applications. Currently, the stateful HBS schemes LMS/HSS and XMSS/XMSS-MT have become standards of IETF, NIST, and ISO, while the stateless HBS schemes SPHINCS+ has become a NIST standard. This course will introduce the state-of-art hash functions and the design principles and security analysis of HBS schemes.