Ian Stewart (Notices, December, 1991) described a computer program, entitled kaos, that was developed at Cornell University by S. Kim and J. Guck-enheimer. We describe here a \second generation" program, called dstool, with much the same functionality as kaos, but a substantially diierent implementation. From the user's point of view, the most obvious diierence is that dstool uses the X11, rather than the SunView, windowing system. However, the distinguishing features of dstool pertain to its modular design. Dstool was built to promote extension and improvement of its capabilities. Dstool (short for Dynamical System TOOLkit) is designed to aid researchers in the investigation of dynamical systems. Toward this goal, our ultimate objective is to construct a comprehensive, open \package" that provides an interactive interface for all computations involving dynamical systems. This objective is ambitious and is certainly not yet fully realized. Sci-entiic libraries of numerical algorithms (e.g., LINPACK, EISPACK, ...) are extraordinarily valuable resources for building complex programs out of modular components. But the value of these individual units is greatly enhanced when they are integrated into a larger software package which uses shared data structures to connect the algorithms with a sophisticated user interface that provides interactive control. Such a package can increase our ability to use computers as tools for gaining insight by minimizing time wasted in repetitious programming and using clumsy schemes for manipulating data. Programs constructed to meet these goals have applications in the classroom 1