The article is devoted to the research of institutes of cooperation and association of territorial communities in Ukraine. The main purpose of the article is to find out the essence of cooperation and association of territorial communities in Ukraine, their characteristics and their relationship, as well as the impact of the completion of community mobilization on the implementation of cooperation.In 2014, the Government of Ukraine launched a large-scale reform of local self-government, which, in particular, provides for the transfer of greater powers from the central government to local governments, expanding their resource base and more. In this regard, the issues of development of cooperation of territorial communities as an effective and flexible tool for ensuring the capacity of territorial communities and sustainable local development are of particular importance. As the experience of other countries shows, the cooperation of territorial communities cannot be considered as a one-time action, but as a set of actions (organizational, legal, institutional, managerial, etc..)The main legal basis for the development of the institute under study was the adoption of a special Law of Ukraine «On Cooperation of Territorial Communities», № 1508 - VII of June 17, 2014 (hereinafter - the Law № 1508 - VII, 2014.
 According to which, the basic concepts of the institution of cooperation, its principles, subjects, organization, forms and procedure for termination are regulated. However, the practical implementation of its instructions has identified an urgent need for a comprehensive study of the institution of cooperation of territorial communities. After all, the latter faced a number of inaccuracies in the conceptual apparatus of cooperation, participation in its organization and lack of public control over its implementation, unclear monitoring by the state and its participation in cooperation, but without specific powers to stop abuses of cooperation, with inability to join already to the existing cooperation, with the general standard forms of agreements on cooperation in which risks of all parties of the agreement are not considered.The undeniable function of this institution is to improve the quality and comfort of members of territorial communities, ie their inhabitants by solving common problems of communities. Along with this, it is necessary to highlight another of its special functions - it is to promote the process of unification of territorial communities, which is traced from foreign experience.But if we analyze the chronology of the stages of decentralization reform, we can see with the naked eye that voluntary unification began simultaneously with cooperation, and how such «promotion» of cooperation in Ukraine did not happen. Instead, we have completely different from foreign functions and influences of these two institutions both on each other and on local self-government in general and territorial communities in particular.The completion of the process of unification of territorial communities and the creation of capable communities has created a number of obstacles to the implementation of the terms of cooperation agreements between their parties. For example, cooperating communities have joined the affluent, indicating their reorganization. Yes, affluent communities have become the successors of real estate, movables, property obligations, but within the balance of the latter. This means that if the successor does not decide to finance cooperation agreements, they will not be able to be completed and implemented in full. Note that the subject of most such agreements is the implementation of joint projects. As a rule, their conditions determine the need to involve contractors who are obliged to pay for the work performed.Based on such consequences of the completion of the process of unification of communities in Ukraine, there is a great need for prompt and comprehensive legislative changes to protect the institution of cooperation of territorial communities and its subjects.