Real-time reactive systems are characterized by their continuous interaction with their environment through stimulus-response behaviour. The safety-critical nature of their domain and their inherent complexity advocate the use of formal methods in the software development process. An effective user interface for real-time reactive systems development environment would hide the complexity of the formalism in the design, and promote ease of use.  This thesis addresses the design and implementation of a Graphical User Interface ( GUI ) which provides precise interaction points for the user-centered tasks of TROMLAB , a rigorous real-time reactive systems development environment being built in the Department of Computer Science, Concordia University.  The scope of this thesis is two-fold. We first reengineer the initial TROMLAB design, which is a prerequisite to GUI development. Reengineering was necessary due to the need for a flexible design with abilities to absorb changing requirements with minimal changes to the design, and usability of the entire TROMLAB environment. We then build an integrated GUI to interact with TROMLAB environment.