The Surface Force Apparatus (SFA) is reviewed. The origin of the technique, from multiple-beam interferometry, tribological studies and measurements of van der Waals forces in air to the versatile instrument of today, is traced. The underlying principles are described while the inherent simplicity of the measurements is stressed. The two chief advantages of the SFA compared to other force-measuring instruments are emphasised-(i) that the zero of separation is accurately determined, and (ii)

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... the shape of the surfaces and the refractive index of the intervening medium can be monitored with multiple-beam interferometry. For these reasons, the SFA is not only a device for measuring forces, it is a valuable tool for studying surface interactions and thin film behaviour in general.