Тhe article is devoted to the study of parliamentary investigation as an institutional mechanism of constitutional control in the system of separation of state power. In the context of the development of modern constitutional States, the importance of effective forms of parliamentary control aimed at ensuring the accountability of the executive branch and compliance with the principles of legality and constitutional governance is increasing. A special place among such mechanisms is occupied by the parliamentary investigation, which is a special parliamentary procedure that allows for an institutional review of the activities of State bodies and officials on issues of public and state importance. The paper examines the theoretical and legal foundations of parliamentary investigation, its place in the system of constitutional control and its role in ensuring the functioning of the mechanism of checks and balances. Based on the comparative legal analysis, the main institutional features of parliamentary investigation and the factors determining its effectiveness in various constitutional systems are highlighted. An institutional model for evaluating the effectiveness of parliamentary investigations is proposed, including criteria such as the regulatory certainty of investigative procedures, the institutional autonomy of Parliament, the scope of powers of parliamentary commissions, and the public nature of investigative procedures.