OptSim is an intuitive modeling and simulation environment supporting the design and the performance evaluation of the transmission level of optical communication systems. With an easy-to-use graphical user interface and lab-like simulation results analysis instruments, OptSim provides a fast learning curve for high usability. The large library of flexible component models and simulation algorithms provide the best trade-off between accuracy and speed to reduce time-to-market and provide cost savings on complex projects and designs. OptSim has been thoroughly validated, demonstrated reliability with a large number of WDM channels, and has a large installed base in research and industrial organizations.