Clinical development of new anticancer drugs can be compromised by a lack of qualified biomarkers. An indispensable component to successful biomarker qualification is assay validation, which is also a regulatory requirement. In order to foster flexible yet rigorous biomarker method validation, the fit-for-purpose approach has recently been developed. This minireview focuses on many of the basic issues surrounding validation of biomarker assays utilised in clinical trials. It also provides an overview on strategies to validate each of the five categories that define the majority of biomarker assays.