<p>This article provides an overview of the athlete's biological passport (ABP) within professional sport, with a particular focus on professional road cycling. We describe the rationale behind the ABP, what is recorded on it (particularly focusing on the haematological passport), what can cause 'natural' variations to it and discuss its role within professional sport. These 'natural' variations often occur at Grand Tours but more research is required into what happens during these multiple-stage road cycling races in terms of the ABP.</p>