An r-sunflower is a collection of sets such that there is a set C (possibly the empty set) called the core, for which every two of the sets have intersection exactly C. Sunflowers are one of the most important structures in extremal set theory since they are the only configuration that one is guaranteed to contain in any sufficiently large collection of k-elements sets. The simple structure of a sunflower enables one to prove many theorems in combinatorics and beyond. This chapter considers the basic theory of sunflowers, culminating in recent breakthroughs.