In view of its complexity, high cost, potential danger, and very specific features, a nuclear power unit needs special handling, especially in the course of its commissioning, during which the fundamentals of its further safe, effective, and economically efficient commercial operation undergo practical checks. The necessity of NPP commissioning is stemming from the presence of risks that can lead to failures and deficiencies in the operation of equipment and systems during the subsequent commercial operation of the power unit, and which should be revealed and removed during the commissioning stage. Two types of risks are commonly encountered in practice. Risks of the first type emerge in constructing and commissioning the power unit, and are associated with the work arrangement and execution processes (process risks). These risks are manageable to a significant extent and can be scheduled. Risks of the second type are associated with nonconformity of the technical quality of work results with the requirements; they are not revealed at the power unit development, design, and construction stages; they are hidden and are only revealed during the tests (technical risks). Since these risks cannot be predicted, they are probabilistic in nature, and the possibility to manage them is significantly limited due to their being hidden in nature. The significance of the commissioning process is stemming from the necessity of full-scale experimental verification of the serviceability and operating conditions for a number of equipment items and systems, which can only be carried out in the course of tests. This can be confirmed on the examples of carrying out physical tests of in-core instrumentation systems, substantiating the seismic safety, dynamic tests. Owing to the possibilities of full-scale tests, the commissioning can also be regarded as a specific process of not only substantiating, but also optimizing the equipment commercial operation conditions and service life. The need of breaking-in the structures in the course of commissioning work should be considered as the third reason of the commissioning process significance. This is because during the commissioning, the maximum number of the existing defects is revealed, and conditions are set up for fully completing the structures breaking-in process to achieve the minimum failure rate levels during the subsequent commercial operation of the NPP