A multielment hoc-film sensor has been developed with closely-spaced elements (2.5mm) in the streamwise direction. Constant temperature anemometers are arranged to monitor only the time varying component of the hot-film element output voltage. In addition to boundary-layer transition detection, the output of these multielement sensors can reveal the locations of boundary-layer separation and reattachment by a 180° phase difference in the 0-200 Hz frequency bandwidth between sensor elements on each side of either the separation or reattachment point. This phase difference phenomenon requires no apriori calibration of the hot-film elements and has been demonstrated on both laminar and turbulent boundary layers. The present paper describes the features of multielement hot-film sensor arrays and discusses results from their application to a number of low-speed and transonic airfoil tests.