Biomarker measurements have become an essential component of oncology drug development, particularly so in this era of targeted therapies. Such measurements ensure that clinical studies are testing our biological hypotheses and can help make the difficult decisions required to choose which drugs to stop developing or de-prioritise. For those drugs taken forward, biomarker measurements may also help choose the appropriate dose, schedule and patient population. In this review we discuss the intrinsic properties of biological sample based efficacy measurements and how these relate to their implementation in oncology drug development by way of points to consider and examples.