The MIB Browser, Operation Definer and Application Component Editor are three prototype TMN application development tools developed at HP Laboratories in Bristol. They were shown at Telecom '95 as part of a demonstration of our belief that appropriate tool support can greatly increase the productivity of the developers of telecoms Operational Support Systems. The three tools take the user from an interactive exploration of the TMN Management Information Base through to the construction of application components. These can be distributed by providing an IDL interface definition which makes the component accessible via CORBA. This article describes the philosophy behind the tools, some details of their implementation and their use in practice.