This publication is part of the outcome of the IAEA's BIOMASS research coordination programme and it describes the work carried out during a four year period from 1996 to 2000 by the BIOMASS Dose Reconstruction Working Group. The objective of this work was to improve models for radiocaesium environmental transport through model testing and intercomparison. The input scenario modelling exercise described in this report provided an opportunity for model testing using post-Chernobyl measurement data. This scenario provided a challenging test in including: a variety of environments, the impact of countermeasures and a follow-up period of 10 years following the accident. Eight participants took part in this exercise and their experience, approaches and models are described. The information and the scenario itself are a valuable resource for other modellers to develop their methodologies further. This publication is primarily intended for use by experts in environmental modelling worldwide, operators of nuclear fuel cycle facilities and regulatory bodies of IAEA Member States.