A bioreactor can be defined as a device that uses mechanical means to influence biological processes. In tissue engineering bioreactors can be used to aid in the in vitro development of new tissue by providing biochemical and physical regulatory signals to cells and encouraging them to undergo differentiation and/or to produce extracellular matrix prior to in vivo implantation. This chapter discusses the necessity for bioreactors in tissue engineering, the numerous types of bioreactor that exist, the means by which they stimulate cells and how their functionality is governed by the requirements of the specific tissue being engineered and the cell type undergoing stimulation.