This document is part of the outcome of the IAEA's BIOMASS research coordination programme and describes the work carried out during a four year period from 1996 to 2000 by the BIOMASS Tritium Working Group (TWG). The objective of this work was to improve models for tritium environmental transport in situations of long-term release of tritium from primary or secondary sources. These situations had not been widely considered before at international level. The TWG developed and analysed six model test exercises and carried out a specifically commissioned field sampling and analysis programme in order to study and understand more fully the behaviour of long-term releases of tritium in the environment. As a result of the work undertaken in the modelling exercises and field sampling programme a number of recommendations are made for modelling tritium behaviour in the environment, data acquisition methods and further work. Appended to this document are summary descriptions of the individual models which participated in the TWG modelling studies, as well as detailed descriptions of each of the modelling scenarios addressed by the Tritium WG. The document is primarily intended for use by experts in environmental modelling worldwide, operators of the nuclear fuel cycle facilities, Regulatory Bodies of the IAEA Member States.