Concern about the quantity of carbon-14 (14C) released for several decades from nuclear facilities has prompted several modelling approaches of 14C behaviour in the environment. The TOCATTA model aims at estimating 14C (and 3H) transfers in terrestrial ecosystems exposed to atmospheric 14C (and 3H) releases from nuclear facilities under normal operating or accidental conditions. The model belongs to the larger framework of the SYMBIOSE modelling and simulation platform that aims at assessing the fate and transport of a wide range of radionuclides in various environmental systems, and their impact on humans and/or biota. In this context, the conceptual and mathematical modelling in TOCATTA have been designed to be relatively simple, minimizing the number of compartments and input parameters required, for being used in an operational mode.