Gastruloids are three dimensional aggregates of embryonic stem cells (ESCs) that, when cultured in specific conditions, exhibit an organization resembling that of an embryo. They develop with three orthogonal axes and contain the primordial cells for various tissues derived from the three germ layers, without the presence of extraembryonic tissues. Notably, they do not possess forebrain, midbrain, and hindbrain structures. Gastruloids serve as a valuable model system for studying mammalian development, including human development, as well as diseases associated with it. They are a model system an embryonic organoid for the study of mammalian development (including humans) and disease.