This chapter focuses on the analysis of the principle of separation of powers in modern states and the role of good faith in the relationships between constitutional state bodies. The first part characterises the determinants of the separation of powers and examines the reasons why referring to the concept of good faith may be desirable in the context of interactions between various constitutional actors. This analysis excludes the relationships between central authorities and local governments, which are discussed in a separate chapter. Subsequently, building on the definition of good faith as an attitude oriented toward achieving constitutional goals and values, the chapter explores its fundamental characteristics and significance for the functioning of the state’s institutional system. In the final section, an attempt is made to determine whether, and to what extent, constitutional good faith in the accepted sense can be enforced – both through judicial means and through extrajudicial mechanisms regulating inter-institutional relations.