This work is motivated by the recent Request For Proposals issued by OMG for a new UML Profile named “Modeling and Analysis of Real-Time and Embedded systems”. The paper describes first some domain concepts for annotating Non-Functional Properties (NFPs), whose focus is on supporting temporal verification of UML-based models. Particular emphasis is given to schedulability and performance analysis for real-time systems. We discuss next some general requirements for NFP annotations and evaluate how the UML profiles for “Schedulability, Performance, and Time Specification” and for “Modeling Quality of Service and Fault Tolerance Characteristics and Mechanisms”, address these requirements. Last but not least, the paper proposes a preliminary framework for describing NFPs by considering the major requirements previously stated and by analyzing some UML mechanisms to attach NFPs to model elements.