Service orientation fosters a high-level model for distributed applications development, which is based on the discovery, composition and reuse of existing software services. However, the heterogeneity among current service-oriented technologies renders the important task of service discovery tedious and ineffective. This dissertation proposes a new approach to address this challenge. Specifically, it contributes a framework supporting the unified discovery of heterogeneous services, with a focus on web, peer-to-peer, and grid services. The framework comprises a service query language and its enacting service discovery engine. Overall, the proposed solution is characterized by generality and flexibility, which are ensured by appropriate abstractions, extension points, and their sup- porting mechanisms. The viability, performance, and effectiveness of the proposed framework are demonstrated by experimental measurements.