A measurement system for a lithographic apparatus includes a sub-frame compliantly mounted on a reference frame. A measurement device, e.g. an alignment sensor, is mounted on the sub-frame. Soft mounting of the sub-frame isolates the alignment sensor from high-frequency disturbances, e.g. acoustic noise, by acting as a low-pass filter with a cut-off frequency, e.g. in the range of from 100 to 200 Hz.