This part aims to identify reasons for the wide differences in budget system laws in advanced countries. Comparative research on budget system laws is virtually non-existent. The economics and public finance literature provides little explanation as to why budget system laws differ so widely across countries. Comparative law studies are more promising, particularly those relating to the hierarchy of law. The differences in the relative importance of the constitution, statutes and regulations provide some explanation as to why countries’ budget-related laws are so different. However, there is no theory to explain why the relative weight of each of these sources of law differs across countries. Political variables are also important, particularly the past and present forms of government. The degree of separation of powers between the executive and the legislature partly explains the extent to which law is used for specifying budget processes and actors.