The purpose of this chapter is to set out the theoretical and doctrinal framework for analysing the concept of good faith in constitutional law. The chapter is divided into four sections. The first section focuses on the main features of good faith in jurisprudence, particularly in public law, as discussed in greater detail in the first part of the book. It notes that although the concept of good faith is typically associated with private law, it also has roots and foundations in the public law system. The following sections examine the existing discourse on good faith in constitutional doctrine. Section two emphasises that the concept has gained prominence in recent years in the face of the crisis of liberal democracy, as a response to instances of the instrumentalisation of the constitution and the abuse of the law to achieve undemocratic ends. Section three shows that the concept of good faith pervades constitutional law, whether it is rooted in the provisions of a particular state’s constitution or emerges from case law. This concept is relevant to the interpretation and application of the constitution in both vertical and horizontal relations. The final section of the chapter undertakes a theoretical exploration of the concept of constitutional good faith, seeking to identify its main characteristics and foundations.