This chapter aims to provide an overview of the wide range of collective effects observed experimentally in colloidal active matter. The diversity of phenomena reported goes along with an increasing number of available experimental systems, sometimes making it challenging to reach a global understanding of these collective effects. To attain this unifying view, we strive to identify similarities across diverse systems by emphasizing their predominant characteristics, particularly focusing on interactions. We first present the collective effects arising from a limited number of swimmers immersed in an assembly of passive particles. Next, we explore the remarkable assemblies resulting from non-swimming spinning particles. We then examine the collective behavior of assemblies of swimmers, encompassing phenomena like self-trapping and flocking, for which a theoretical description of experimental observations is arguably the most advanced. Finally, we discuss more effects stemming from long-range chemical interactions or non-reciprocal interactions, which are more complex to address theoretically. There, the further progress toward a unified understanding implies better experimental characterization of systems properties, especially interactions, as well as the development of theoretical frameworks integrating these complex effects.