Improved performance in many optical instruments has been achieved by considering coherence effects in the design of the optical instrument. In this paper such improvements are reviewed by heuristically describing the fundamental principles of partial coherence which are required in the design of such instruments. Furthermore, these coherence effects, which arise from both coherent and incoherent sources of radiation, are illustrated by discussing a selected set of instruments in which improved performance has been achieved. These examples include high-resolution recording and analyzing instruments which use incoherent sources, as well as imaging and mensuration instruments which use coherent sources. A set of guidelines for determining when coherence effects influence system performance with respect to linearity, resolution, and noise is also presented.