The mechanisms that regulate mammalian organ size are poorly understood. It is unclear whether the pathways that control organ size also impinge on stem/progenitor cells. A highly expressed gene in stem cells is YAP1 [1], the ortholog of Drosophila Yorkie, a downstream component of the Hippo pathway [2]. Mutations in components of this pathway produce tissue overgrowth phenotypes in the fly whereas mammalian orthologs, like salvador [3], merlin [4], LATS [5], and YAP1 [6, 7], have been implicated in tumorigenesis. We report here that YAP1 increases organ size and causes aberrant tissue expansion in mice. YAP1 activation reversibly increases liver size more than 4-fold. In the intestine, expression of endogenous YAP1 is restricted to the progenitor/stem cell compartment, and activation of YAP1 expands multipotent undifferentiated progenitor cells, which differentiate upon cessation of YAP1 expression. YAP1 stimulates Notch signaling, and administration of γ-secretase inhibitors suppressed the intestinal dysplasia caused by YAP1. Human colorectal cancers expressing higher levels of YAP1 share molecular aspects with YAP1-induced dysplastic growth in the mouse. Our data show that the Hippo signaling pathway regulates organ size in mammals and can act on stem cell compartments, indicating a potential link between stem/progenitor cells, organ size, and cancer.