Organogenesis, tissue homeostasis and organ function involve complex spatial cellular organization and tissue dynamics. The underlying mechanisms of these processes, and how they are disrupted in disease, are challenging to address in vivo and ethically impossible to study in human. Organoids, three-dimensional (3D) stem cell cultures that self-organize into ex vivo 'mini-organs', now open a new window onto cellular processes within tissue. Light microscopy is a powerful approach to probe the cellular complexity that can be modeled with organoids. This combination of tools is already leading to exciting synergies in stem cell and cancer research.