A sunflower, or a \(\Delta \) -system is a collection of sets such that the intersection of any two sets coincides with the intersection of all sets. The topic of this survey is the \(\Delta \) -system method in extremal set theory. It originated in the 1970s from the work of Erdős, Deza and Frankl and was developed by Frankl and Füredi in the 1980s. Roughly speaking, it comprises several approaches of how to analyze a family of sets by looking at the \(\Delta \) -systems, or sunflowers, that it contains. In this survey, we cover the method and its various applications.