Tissue formation within the body, as part of a development or repair process, is a complex event in which cell populations self-assemble into functional units. There is intense academic, medical, and commercial interest in finding methods of replicating these events outside the body. This interest has accelerated with the demonstration of the engineering of skin and cartilage tissue in the laboratory and there is now worldwide activity in the in vitro regeneration of tissues including nerve, liver, bone, heart valves, blood vessels, bladder, and kidney. Approaches to tissue engineering center on the need to provide signals to cell populations to promote cell proliferation and differentiation. This review considers recent advances in methods of providing these signals to cells using examples of progress in the engineering of complex tissues.