This lecture focuses on a very common measurement problem: the measurement of a small differential signal in the presence of a much larger, undesired common-mode signal. This problem is introduced in context of the Wheatstone bridge, which is used for the readout of resistive sensors. To amplify differential voltages, the difference amplifier and the three-opamp instrumentation amplifier are introduced. Their ability to reject common-mode input signals is discussed and quantified in terms of the common-mode rejection ratio (CMRR).