To assess the quality of critical information infrastructure systems throughout the entire life cycle, quantitative assessments of the quality of the software / digital system are required. The article presents quality models in use and quality models of a software / digital system and a diagram of threats to the quality of software / digital systems and methods for their prevention. The quality model of a software / digital system considers in detail the allocation of a 3-level structure (factor, criterion, metric). Recommendations are given on using evaluation characteristics for different life cycle stages, a structural-functional model of quality assessment present. The presented approach is used to assess software quality for critical information infrastructure systems (nuclear power facilities, transport infrastructure, etc.).