The paper has analyzed the academic heritage of Prof. Ekaterina I. Kozlova in the part of her research of fundamental issues of the theory of constitutional (state) law, namely: the system of the branch under consideration and public law institutions. The author focuses on suggested by Prof. Ekaterina I. Kozlova innovative division of the system of the branch of constitutional law and the structure of the Constitution, the substantive approach to the construction of the institution of state law, consistent defense of the concept of unity of the system of constitutional (state) law. The author has determined the current significance of the conclusions made by Prof. Ekaterina E. Kozlova concerning the development of constitutional law institutions: complication of their structure, expansion and specialization of constitutional law norms and, as a consequence, the emergence of complex institutions and the problem of subsectors; an increase in the number of norms, i.e. principles and procedural rules in constitutional law.