Simplicial sets form a classical and well known tool for modelling topological spaces, and more recently topologically enriched categories and infinity categories. I will present an extension of the category of simplicial sets, called “dendroidal sets”, and explain how these can model topological operads and their algebras. The definition is based on a simple category of trees, and the goal of the talk will be to give a leisurely introduction to dendroidal sets and some of their uses. Applications include an efficient infinite loop space machine, the analysis of derived mapping spaces of E_n operads, and Koszul duality, for example.

The talk will be based on work with or by many people, among whom Boavida, Cisinski, Goeppl, Heuts, Hinich, Weiss, and others.