This chapter focuses on extracorporeal devices. Specific attention is given to describing the various fluid contacting schemes that are used as well as on estimating mass transfer coefficients and the solute diffusivity in blood. Design methods for hemodialysis, peritoneal dialysis, and aquapheresis are described. Also presented are design methods for blood oxygenators. This is followed by a discussion on immobilized enzyme reactors and their applications. Enzyme reaction kinetics and the effect of intraparticle diffusion on the observed enzyme reaction rate are described. Reactor design methods for the packed bed reactor and the well-stirred reactor are also presented. Affinity adsorption is also described.