This paper describes the objectives, rationale, and content of a real-time UML profile that is currently in the process of being adopted as a standard technology by the Object Management Group. The profile first defines a general framework for describing quality of service (QoS) attributes of elements of the modeled system. This framework is then used to define three specialized component profiles: a general profile for analyzing real-time systems for schedulability using standard schedulability analysis techniques, a specialized version of that profile intended specifically for Real-Time CORBA technology, and, finally, a profile for analyzing the performance characteristics of systems based on queuing theory. These profiles are designed with a view towards automating the analysis process through the exchange of models and analysis results between model editing tools and specialized model analysis tools. The intent is to make design of real-time software systems more reliable and more efficient through the use of predictive models, a technique that is fundamental in more mature engineering disciplines.