A method for use by a first user terminal, the first user terminal comprising a processor and a first application to be executed by said processor, the first application being configured to cause the processor to delegate execution of a first service to a further, different, application, the method comprising: determining that said first service is to be executed on behalf of the first application; responsive to said determination, sending, to a data store (304b, 108. 314), a request message comprising an identifier of said first service, the data store comprising entries (500, 502) indicating a plurality of applications held on one or more user terminals, and indicating one or more services that may be executed, on request, by a corresponding application of the plurality of applications; receiving a response message from the data store, the response message identifying a second application and a third application, different to the second application, held on the one or more user terminals to which execution of said first service may be delegated; selecting said second application in preference to said third application in the case that the second application is determined to be held on the first user terminal and the third application is determined not to be held on the first user terminal; or selecting said second application in preference to said third application in the case that an operating system of a second user terminal, different from said first terminal, hosting the second application is determined to match with an operating system of the first user terminal, and the operating system of a third user terminal, different from said first user terminal and said second user terminal, hosting the third application is determined not to match with the operating system of the first user terminal; or selecting said second application in preference to said third application in the case that it is determined that a second user terminal, different from said first terminal, hosting the second application is located closer to the first user terminal than a third user terminal, different from said first user terminal and said second user terminal, hosting the third application; and causing the processor to delegate execution of said first service to said selected second application. determining that said first service is to be executed on behalf of the first application; responsive to said determination, sending, to a data store (304b, 108. 314), a request message comprising an identifier of said first service, the data store comprising entries (500, 502) indicating a plurality of applications held on one or more user terminals, and indicating one or more services that may be executed, on request, by a corresponding application of the plurality of applications; receiving a response message from the data store, the response message identifying a second application and a third application, different to the second application, held on the one or more user terminals to which execution of said first service may be delegated; selecting said second application in preference to said third application in the case that the second application is determined to be held on the first user terminal and the third application is determined not to be held on the first user terminal; or selecting said second application in preference to said third application in the case that an operating system of a second user terminal, different from said first terminal, hosting the second application is determined to match with an operating system of the first user terminal, and the operating system of a third user terminal, different from said first user terminal and said second user terminal, hosting the third application is determined not to match with the operating system of the first user terminal; or selecting said second application in preference to said third application in the case that it is determined that a second user terminal, different from said first terminal, hosting the second application is located closer to the first user terminal than a third user terminal, different from said first user terminal and said second user terminal, hosting the third application; and selecting said second application in preference to said third application in the case that the second application is determined to be held on the first user terminal and the third application is determined not to be held on the first user terminal; or selecting said second application in preference to said third application in the case that an operating system of a second user terminal, different from said first terminal, hosting the second application is determined to match with an operating system of the first user terminal, and the operating system of a third user terminal, different from said first user terminal and said second user terminal, hosting the third application is determined not to match with the operating system of the first user terminal; or selecting said second application in preference to said third application in the case that it is determined that a second user terminal, different from said first terminal, hosting the second application is located closer to the first user terminal than a third user terminal, different from said first user terminal and said second user terminal, hosting the third application; and causing the processor to delegate execution of said first service to said selected second application.