As nearly all imaging has moved from analogue media such as photographic film into digital image files it has become vital to understand how resolution and digital sampling affect the information contained in these images. This article is a basic introduction to the important concepts and how they should be applied, using general assumptions, rather than a rigorous mathematical approach. The focus is on optical microscopy although the essentials are the same for any digital imaging. The basic concepts are explained with reference to optical microscopy, but then the article moves on to more general aspects of sampling in digital imaging. More detailed, and mathematical, treatments can be found in a range of literature but a particularly detailed and instructive description can be found in Chapter 4, Handbook of Biological Confocal Microscopy, 3rd edition, Ed J.B. Pawley, Springer 2006.