Brain organoids and neural organoids are more than cell aggregates consisting of a single cell type induced from ES or iPS cells; they are three-dimensional tissue structures that contain multiple cell types and partially recapitulate the architecture of the brain and neural tissue. The ability to observe the process of human brain formation in vitro has opened the way to research on neurogenesis in the human fetal period and on pathological conditions such as disease-specific signaling abnormalities. This section introduces the fundamentals of brain organoids and neural organoids, including the principles underlying their generation, differentiation induction protocols, and the reagents used for their production.