InAs-GaSb strained layer superlattices (SLSs) form a narrow band gap material whose cut-off wavelength can be tuned from 3 um to beyond 30 um. Theory predicts that in the LWIR and VLWIR, the SLS narrow bandgap layer structures can be engineered to reduce Auger recombination, relative to other narrow bandgap materials, such as HgCdTe. This should result in the SLS diodes having better performance than currently available detectors. A key to achieving this improved performance is knowing the detailed layer structure of the superlattice, and being able to accurately model this layer structure. Having an accurate model to guide the improved performance is essential to optimizing this material system. Cross-sectional scanning tunneling microscopy data will be presented which shows that the actual layer structure differs significantly from the intended layer structure, due to the detailed dynamics of MBE growth and the very thin layers in the superlattice. Specifically, cross-sectional scanning tunneling microscopy demonstrates that the InAs contains excess antimony, and the GaSb excess indium, due to segregation from the underlying arsenide-on-antimonide, or antimonide-on-arsenide, heterojunctions respectively. These deviations from the intended structure have a significant impact on the predicted properties of the superlattice. The predicted behavior of the intended and actual superlattice structures will be compared to measured performance.