— Robots are increasingly being used in Computer Integrated Surgery (CIS) systems, yet to our knowledge, there is no open source software that is specifically targeted at this application domain. In this paper, we derive unique requirements for medical robot controllers that are based on our experiences in the field. We present an overview of the second-generation open source software package (the cisst package), currently under development, that will include a family of application frameworks with dynamically loaded software components. We then focus on some some key design details, which include extensive application of the command pattern as well as a dynamic interface query mechanism, and present a simple example to illustrate the concepts.