The Individual Behaviour Model (IBM) GITM (General Individuals Transport Model) code is an off-line particle tracking model that requires velocity fields from a hydrodynamic model. It includes physical particle advection and diffusion, and biological development and behaviour. This method ensures that particles follow stream lines exactly. Furthermore, a random walk method with advective correction was included to simulate diffusion. The biological development and behaviour module of GITM allows particles to progress through a user-defined number of egg and larval development stages. Each stage allows for separate formulations and settings for growth, mortality and vertical migration behaviour.