In this expository talk, I will give an overview of Greg Hjorth's theory of turbulence and the problem of classifying by countable structures. I will begin by a review of Scott sentences of countable structures, and then show how a similar process can be carried out for arbitrary Polish group actions, resulting in a coarsening of the orbit equivalence relation which we will call "Hjorth isomorphism". I will then explain how to understand turbulence by examining the relationship between these two equivalence relations. The material in this talk is mainly due to Hjorth, with some contributions by Kechris and more recently by Lupini and Panagiotopoulos.