This chapter studies the important role of financial assistance in furthering environmental protection. Financial assistance is composed of a complex mosaic of financial mechanisms and sources, which are linked to and affected by global conventions, multilateral institutions, bilateral aid, and private sector investment. Moreover, the principle of common but differentiated responsibilities has provided a specific rationale for the provision of financial assistance within multilateral environmental agreements (MEAs) and through various financial mechanisms. Through the provision of financial assistance, developed countries contribute to covering the incremental costs of protecting the global environment incurred by developing countries. The functions of financial assistance include, inter alia, capacity-building, disaster relief, financing of pollution control equipment, and compliance assistance. Among these various targets, capacity-building has received an important political emphasis. The chapter then outlines the different types and sources of financial assistance, as well as its aim and nature, before analysing the legal structure of financial mechanisms.