In this talk we will describe state-of-the-art approaches to the modeling of strain relaxation and dislocation dynamics in ZnSSe/GaAs (001) heterostructures. Current modeling approaches are all based on the extension of the original Dodson and Tsao plastic flow model to include compositional grading and multilayers, dislocation interactions, and differential thermal expansion. Important breakthroughs in the last twenty-four months have greatly enhanced the utility of these modeling approaches in three respects: i) pinning interactions have been included in graded and multilayered structures for the first time, providing a better description of the limiting strain relaxation as well as the dislocation sidewall gettering; ii) a refined model for dislocation-dislocation interactions including jogging provides a more accurate physical description of compositionally-graded layers and step-graded layers; and iii) inclusion of back-and-forth weaving of dislocations provides a better description of dislocation dynamics in structures containing strain reversals, such as strained-layer superlattices or overshoot graded layers. We will describe these three key advances and illustrate applications of each to practical heterostructures.