Tissue engineering is a rapidly growing field in biotechnology. The use and packaging of synthetic materials, biologic compounds, and cellular components of specific tissues can be envisioned to replace physiologic function of diseased organs. Long-term ex vivo therapy for kidney failure has been achieved, so that the kidney may be the first solid organ in which tissue engineering concepts can produce an implantable device for long-term in vivo replacement therapy. To replace the kidney’s excretory function, an implantable bioartificial kidney requires both a device to replace blood ultrafiltration performed by renal glomeruli and a device to replace transport regulatory function of the renal tubule. The initial concepts for these devices are just beginning to be considered and developed.