The Image-Guided Software Toolkit (IGSTK: pronounced IGstick) is a high-level open-source component based software toolkit. The toolkit is currently under development and is intended to make it easy for researchers to prototype and develop image-guided surgery applications. The toolkit has been used ddto develop an example application incorporating electromagnetic tracking of a surgical needle. This paper presents the development approach, the requirements defined, the components identified and the example application. (C) 2004 Published by Elsevier B.V.