The directed energy modeling and simulation community can make important direct contributions to the joint warfighting community by establishing clear and fully integrated future program requirements. These requirements are best determined via analysis of the expected variability/uncertainty in system performance arising from spatial, spectral and temporal variations in operating conditions. In this study of atmospheric effects on HEL systems, the parameter space is explored using the Air Force Institute of Technology Center for Directed Energy's (AFIT/CDE) High Energy Laser End-to-End Operational Simulation (HELEEOS) parametric one-on-one engagement level model. HELEEOS is anchored to respected wave optics codes and all significant degradation effects-including optical turbulence and molecular, aerosol, and liquid water drop/droplet absorption and scattering-are represented in the model. Beam spread effects due to thermal blooming caused by the various absorbers are considered when appropriate. Power delivered in a 5 cm diameter circular area normalized by the total transmitted power is the primary performance metric used in the study, with results presented in the form of histograms. The expected performance of laser systems operating at both low and high powers is assessed at 24 wavelengths between 0.355 &mgr;m and 10.6 &mgr;m for a number of widely dispersed land and maritime locations worldwide. Scenarios evaluated include both up and down looking generally oblique engagement geometries over ranges up to 6000 meters in which anticipated clear air aerosols and thin layers of fog, and very light rain are simulated. Seasonal and boundary layer variations (summer and winter) for nighttime conditions for a range of relative humidity percentile conditions are considered to determine optimum employment techniques to exploit or defeat the environmental conditions. Each atmospheric particulate/obscurant is evaluated based on its wavelength-dependent forward and off-axis scattering characteristics and absorption effects on laser energy delivered. In addition to realistic vertical profiles of molecular and aerosol absorption and scattering, correlated optical turbulence profiles in probabilistic (percentile) format are used, a feature unique to HELEEOS.