This document discusses email spam filtering. It begins by introducing email and the problem of spam. It then describes the two main types of spam filtering: server-side filtering that occurs before emails reach the user, and client-side filtering using user-created rules. Some techniques used by both types are discussed such as block lists and virus definitions. The document then focuses on Gmail spam filtering, how messages are marked as spam, and reasons a message may be placed in the spam folder like unconfirmed senders or messages previously marked as spam. Advantages and disadvantages of spam filtering are provided.