During adult homeostasis and regeneration, the freshwater planarian must 
accomplish a constant balance between cell proliferation and cell death, while 
also maintaining proper tissue and organ size and patterning. How these ordered 
processes are precisely modulated remains relatively unknown. Here we show that 
planarians use the downstream effector of the Hippo signaling cascade, yorkie 
(yki; YAP in vertebrates) to control a diverse set of pleiotropic processes in 
organ homeostasis, stem cell regulation, regeneration and axial patterning. We 
show that yki functions to maintain the homeostasis of the planarian excretory 
(protonephridial) system and to limit stem cell proliferation, but does not 
affect the differentiation process or cell death. Finally, we show that Yki acts 
synergistically with WNT/β-catenin signaling to repress head determination by 
limiting the expression domains of posterior WNT genes and that of the 
WNT-inhibitor notum. Together, our data show that yki is a key gene in 
planarians that integrates stem cell proliferation control, organ homeostasis, 
and the spatial patterning of tissues.