WOSP is a forum for researchers interested in the intersection of Software Engineering and Performance Evaluation. Software Performance Engineering (SPE) research aims to build predictable performance into software by specifying and analysing quantitative behaviour from the early development phases of a system throughout its entire life cycle. Analysis may be based on various kinds of modelling, design and measurement tools. Modern software systems are complex (distributed over a wide area, mobile, running on heterogeneous platforms, belonging to different administrative domains) and often have strict performance and scalability requirements. This workshop brings together software engineers, developers, performance analysts and modellers who address the challenges of increasing system complexity, rapidly evolving software technologies, short time to market, incomplete documentation, and less than adequate quantitative models and tools. We expect a strong mixture of academic and industrial participation. The conference will be held in Palma, Spain on 11-15 July 2005. Further information including the complete call for papers can be found at <http://wosp2005.uib.es/pre.htm> http://wosp2005.uib.es/pre.htm .