Self-organization is ubiquitous in biological systems at all scales, from the animal world down to the intra-cellular environment. In all these systems, the dynamics of micro-constituents can lead to emergent large-scale behaviours, such as static patterns or collective motion. Active matter provides a solid framework to explain the physics behind these processes at many different scales. So far, the literature on the subject has mostly focused on single-component active systems; nonetheless, monodispersity imposes strong limitations on the complexity of the emergent macroscopic phases. In order to obtain less idealised self-assembling structures like the ones encountered in biology, heterogeneity must be included.