I am a developmental biologist. Over the last fifteen years my work has focused on using Embryonic Stem Cells (ESCs) to explore cell fate decisions during mammalian development. Recently, we have developed a novel experimental system which is changing the way to study early mammalian development. Defined numbers of mouse ESCs are aggregated in controlled culture conditions and over time develop structures that mimic the organization of the vertebrate embryo; we call them ‘gastruloids’ because they mirror the process of gastrulation. This system is opening up unforeseen experimental possibilities for studies of mammalian development whose embryos, particularly at the crucial early stages, are difficult to access. Recently we have successfully extended this work to human ESCs and obtained human gastruloids which mimic the postgastrulation body plan thus opening up the possibility to, for the first time, study the process of gastrulation and its consequences in humans.