This paper introduces Graphical Profile Definition Language (GPDL), a language for the definition of profiles for alerting systems, and an interactive system with which users can construct and edit GPDL-based profiles. In an alerting system profiles define potentially complex conditions about which users wish to be notified. Most current approaches to supporting profile definition, such as those that require the use of XML/XPath, are unsuitable for the general user population. Others support only a limited set of possible profiles to reduce the complexity of the interface. GPDL overcomes these limitations, supporting users to graphically express arbitrarily complex profiles involving Boolean expressions and temporal constraints. The GPDL editor provides a direct manipulation environment in which profiles can be constructed in a dynamic and flexible manner. The paper also presents the findings of a user evaluation of the GPDL language and editor. With minimal training users were able to interpret and specify profiles with high levels of accuracy and had positive subjective responses to the language and user interface. Some aspects that were not readily understood by all users, such as absolute and repetitive temporal constraints, are discussed along with suggested future work.