The cytopathology of intraocular lens (IOL) following implantation is reviewed. A newly placed implant attracts macrophages and these settle on its surfaces to form optically clear membranes composed of so-called fibroblast-like cells and a film of proteinaceous material. The membranes apparently become tougher and more firmly adherent with time. The fibroblast-like cells have phagocytic abilities and may form fibrous structures on the surface of implants in complicated situations. Few multinucleated giant cells may be seen in successful cases, but great numbers of giant cells are indicators of adaptation problems. The giant cells on implants include foreign-body, Touton, and Langhans' types. Giant cells on lens implants are commonly associated with nongranulomatous inflammatory cell infiltration in the iris and ciliary body. Inflammatory reactions to IOLs appear to be related to the intolerance of the eye to the IOL foreign body. I present evidence that such inflammatory reactions are a limiting factor in the success of IOL implantation.