During development, diverse cellular identities are established and maintained in the growing animal. Although remarkably robust in the body, cellular identities can be changed using experimental techniques. Lineage reprogramming is an emerging field at the intersection of developmental and stem cell biology in which a adult cells are stably reprogrammed into a distinct type of cell by forced expression of specific genes . Lineage reprogramming enables the direct conversion of readily available cells from patients (such as skin cells) into disease-relevant cell types (such as brain cells and heart muscle cells) or into cells that resemble embryonic stem cells. Although remarkable progress has been made towards developing new reprogramming methods, cellular reprogramming remains difficult and needs to be improved in order increase the translational utility of reprogrammed human cells. In this review, we will discuss how transcription factors induce changes in cellular identity in the context of animal development and in cellular reprogramming.