With the introduction of modern aircraft utilizing laminar flow, flow visualization has become an important diagnostic tool in determining aerodynamic characteristics such as surface flow direction and boundary-layer state. Oil flow and sublimating chemical techniques are discussed, and the use of sublimating chemicals is examined in detail. Oil is used to visualize boundary-layer transition location, shock-wave location, regions of separated flow, and surface flow direction. Sublimating chemicals are used to visualize both the location and mode of boundary-laye r transition. The different modes of transition are characterized by different patterns in the developed sublimating chemical coating. The discussion includes interpretation of these chemical patterns and the temperature and velocity operating limitations of the chemical substances. Information for selection and application of appropriate chemicals for a desired set of flight conditions is provided.