New technologies have led to the development of an increasing number of targeted therapies and interest in combining these with conventional therapy to provide individualised patient treatments. New drug or treatment regimens must, however, undergo rigorous testing under strictly controlled conditions before they can be adopted as standard. This can be expensive, time-consuming and inefficient. Surrogate end-points have been proposed as an alternative, which could be measured earlier or more conveniently than true end-points. The aim of this paper is to review the definition, advantages, disadvantages and potential pitfalls of biological surrogate end-points in the context of cancer treatment.