Software designers are motivated to utilize off-the-shelf software components for rapid application development. Such applications are expected to have high reliability as a result of deploying trusted components. The claims of high reliability need further investigation based on reliability analysis techniques that are applicable to component-based applications.This paper introduces a probabilistic model and a reliability analysis technique applicable to high-level designs. The technique is named "Scenario-Based Reliability Analysis" (SBRA). SBRA is specific for component-based software whose analysis is strictly based on execution scenarios. Using scenarios, we construct a probabilistic model named "Component-Dependency Graph" (CDG). CDGs are directed graphs that represent components, component reliabilities, link and interface reliabilities, transitions, and transition probabilities. In CDGs, component interfaces and link reliabilities are treated as first class elements of the model. Based on CDGs, an algorithm is presented to analyze the reliability of the application as the function of reliabilities of its components and interfaces. A case study illustrates the applicability of the algorithm. The SBRA is used to identify critical components and critical component interfaces, and to investigate the sensitivity of the application reliability to changes in the reliabilities of components and their interfaces.