Skin flaps are classified as random, axial, musculocutaneous, or fasciocutaneous according to the anatomy by which the skin receives its blood supply. The delay phenomenon allows larger and more reliable skin flaps to be transferred but requires additional surgical procedures. Efforts to mimic the delay phenomenon by pharmacologic means have, unfortunately, been disappointing. The major recent advances in flap design and reliability have come from increased knowledge of the anatomy of skin circulation, specifically musculocutaneous and fasciocutaneous circulation. In an effort to accurately predict flap survival, various monitoring techniques have been tried. These include visual inspection of fluorescein, fluorescein photography, dermofluorometry, surface temperature readings, photoplethysmography, ultrasound Doppler flowmeters, laser Dopplers, and transcutaneous PO2 monitors. Fluorescein has proved to be consistent and reliable, and the dermofluorometer has the advantage of using smaller fluorescein doses that can be frequently repeated. The other monitoring devices each have additional advantages and disadvantages.