The Lagrangian model JETLAG is presented in detail in this chapter. The model predictions are confirmed by extensive laboratory experiments as well as field data of ocean outfalls worldwide. It has been further developed by incorporating three-dimensional computer graphics techniques. The new VISJET software package is a PC-based interactive model that gives virtual reality displays of the predicted plume. The buoyant jets discharged in multi-directions interact with the crossflow to form a complex three-dimensional pattern. The predicted concentration and dilution can be obtained directly simply by pointing the cursor at the location where quantitative information is desired. When the receiving water is density-stratified, VISJET depicts the predicted trapping level of the plume and spreading of the plume fluid in a horizontal layer. The interrogative computer graphics also provides the determination of the degree of merging between the bent-over jets from a multi-jet group in any downstream cross-section plane. The composite dilution of the jet group can hence be determined. The versatility and power of the simple VISJET model is illustrated by a case study of a major ocean outfall of the Hong Kong Harbour Area Treatment Scheme (HATS). Important practical issues relevant to modern environmental engineering are also included: dense jets, sediment jets and bubble plumes, the role of Computational Fluid Dynamics (CFD) models, and the coupling of the near field plume model and shallow water circulation models in the “intermediate” and “far field”.