The purpose of this report is to compare individual pupil achievement on two high profile sets of assessments in England: the Programme for International Student Assessment (PISA) and the General Certificate of Secondary Education (GCSE). The research in this report will add to well-documented differences between PISA and GCSEs to look at the strength of association between achievements on these two tests from an empirical perspective. A small amount of existing research has shown a relationship between PISA and GCSE within particular subjects (DfE, 2017). Specifically, the Department for Education's (DfE's) report compares achievement in PISA reading, maths and science with performance in the GCSE English, maths and science respectively. This report goes beyond these published statistics by examining correlations of the PISA domains with a range of GCSE subjects, and also by benchmarking against correlations between GCSEs and scores from key stage 2 (KS2) tests taken at the end of primary education. As part of this process, the report explores some of the technical issues that arise when attempting to link the various sources of data. Finally, and for the first time, this report shows how performances in the different PISA domains relate to the subject choices that pupils make at GCSE. [For the executive summary to this report, "The Link between Subject Choices and Achievement at GCSE and Performance in PISA 2015: Executive Summary," see ED600661.]