Constitutional law is a sprawling subject, encompassing many topics of legal–philosophical interest. This chapter addresses several of the most fundamental and widely discussed issues, organised into three clusters. First, what is constitutional law and what are its necessary or central features or functions? Second, what is the proper role for the judiciary in enforcing constitutional law? Third, how should constitutions be interpreted? Under these headings, the chapter examines, among other things: the puzzle of ‘unwritten’ constitutions, differences between constitutional ‘conventions’ and constitutional law, and the relationship between theories of constitutional interpretation and general jurisprudential accounts of the nature or contents of law. Most issues that especially concern constitutional rights lie beyond the scope of this chapter.