I first review positive-operator and projection valued measures in quantum mechanics, and then address a few applications in quantum key distribution (QKD). The positive operator valued measure (POVM) is useful in the design of a quantum key receiver. The projection valued (PV) measure is useful in the design of QKD eavesdropping devices. In this case the measurement determines correlations with the measurements made by the lefitimate receiver and therewith the maximum information gain by the probe. It is essential to know the latter for privacy amplification.