IPD meta-analyses have an established history in cardiovascular disease and cancer, where the methodology has been developing steadily since the late 1980s. In cancer, for example, there are now more than 50 IPD meta-analyses of screening and treatment across a wide range of solid tumour sites and haematological malignancies (Clarke 1998). IPD have also been used in systematic reviews in many other fields (Simmonds 2005), including HIV infection, dementia, epilepsy, depression, malaria, hernia and asthma. The Cochrane Collaboration Individual Patient Data Meta-analysis Methods Group web site includes a database of ongoing and completed IPD reviews where further information can be found (see Box 18.6.a).