the f-invariant was introduced by lewis bowen in 2008 and is a real-valued isomorphism invariant that is defined for a large class of probability measure-preserving actions of finite-rank free groups. most notably, the f-invariant provided the first classification up to isomorphism of bernoulli shifts over finite-rank free groups. it is also quite useful for the study of finite state markov chains with values indexed by a finite-rank free group. the f-invariant is conceptually similar to entropy, and it has a formal connection to sofic entropy. in this expository talk, i will introduce the f-invariant and discuss some of its basic properties.