This chapter compares the extent to which law is used to specify budget players and processes, with a particular focus on 13 OECD countries. Although budget system laws serve differing purposes, new budget-related laws are often adopted to introduce budget reforms. Differences in the laws underpinning the main budget players – especially legislatures, the political and non-political executive, and external audit offices – are found to be wide. Important differences are observed in the extent to which law is used for specifying each stage of the budget processes: preparation, presentation, adoption (by the legislature), execution, government accounting and fiscal reporting arrangements. Comparative “models” or tables are provided for various budget-related issues and processes, including: budget timetables (legal requirements); fiscal rules; medium-term budget frameworks; the nature, structure and duration of appropriations; annual and in-year accounts, as well as for specific budget players such as parliamentary budget committees or external audit offices.