The key to obtaining valid results from a large, international survey is having access to assessment items that are fit for the intended purpose. They must align with and incorporate the requirements of the relevant framework, give students fair and reasonable opportunity to demonstrate their true level of performance, cover a wide range of student abilities and mathematical literacy content, and work well in many different languages and cultural contexts. This chapter describes in detail the process that item writers from the PISA international contractors applied to generate items for the 2012 survey, from initial draft to final assessment, for both paper-based and computer-based items.