The precision and high dimensionality of the Gaia catalogue allows us to identify groups of co-eval, co-moving stars. This dataset has enabled the discovery of hundreds of clusters, and has revealed significant amounts of substructure in young stellar complexes. After presenting some of the tools and techniques recently applied to the Gaia DR2 data, I will use the example of the Vela-Puppis region to show that a continuous and arbitrary transition exists between the aggregates traditionally called "associations" and "clusters".