ComposIT is an online web service composition tool developed by the Intelligent Systems Group (GSI) of the University of Santiago de Compostela. This web application is a front-end of a service which implements a heuristic-based search algorithm¹⁾ that finds an optimal composition given a user request. The algorithm was successfully validated using eight datasets provided by the WS-Challenge 2008. These datasets have a variable number of services (from 158 to 8119) and the inputs and outputs are semantically described. To start the web application, please follow this link http://www.gsi.dec.usc.es/composit