Ambient Assisted Living (AAL) systems for the ageing and cognitively disabled do not exist in isolation. What characterizes such systems is the cooperation of several different stakeholders in the care process. The healthcare systems used by these stakeholders throughout Europe use proprietary data formats and some systems are designed to interoperate with other systems based on standardized data formats, such as HL7, CEN TC251, or national standards. Consequently an interoperability solution needs to be able to communicate with external systems in an agreed upon format. This paper reports on our work in the EU IST MPOWER project where we have designed and implemented a pluggable interoperability mechanism based on Patterns, Service Oriented Architectures, Web Services and XSDL transformations. The services we present are freely available as open source under the MIT license and a subset is proposed for standardization in the Healthcare Service Specification Project (HSSP).