The main features of creating an information model (Digital Twin) for a technically complicated item are formulated. These include: a multitude of systemic representations of the item, individualization of its behavior and state under specific operating conditions, fatal uncertainty in the description of component properties, reconstruction of data based on sensors readings, individual lifetime expense in operation. Above features are illustrated by authors’ developments in application to power transmissions. The key issues of synthesis, calculation, design and diagnostics of transmissions based on the created complex of lifetime-and-functional models are solved. Transmission is considered as a multicomponent system with a variable structure and states. Creating a single universal model to reproduce basic properties of the technically complicated item is impossible. Principles of constructing models and methods for calculating, monitoring and predicting functional and lifetime properties are described. The basic representations of the transmission (structure, regular mechanical system, kinematic and dynamic diagrams, diagnostic model, schemes of limiting states, lifetime expense model) and the mathematical models serving them are developed. The approaches considered are methodologically typical for complex mechanical and combined objects based on mechanical systems.