\'Etale groupoids and their $C^*$-algebras

These notes were written as supplementary material for a five-hour lecture series presented at the Centre de Recerca Mathem\`atica at the Universitat Aut\`onoma de Barcelona from the 13th to the 17th of March 2017. The intention of these notes is to give a brief overview of some key topics in the area of $C^*$-algebras associated to \'etale groupoids. The scope has been deliberately contained to the case of \'etale groupoids with the intention that much of the representation-theoretic technology and measure-theoretic analysis required to handle general groupoids can be suppressed in this simpler setting. A published version of these notes will appear in the volume tentatively titled "Operator algebras and dynamics: groupoids, crossed products and Rokhlin dimension" by Gabor Szabo, Dana P. Williams and myself, and edited by Francesc Perera, in the series "Advanced Courses in Mathematics. CRM Barcelona." The pagination of this arXiv version is not identical to Birkh\"auser's style, but I have tried to make it close. The theorem numbering should be correct. I'm grateful to the CRM and Birkh\"auser for allowing me to post a version on arXiv.