This publication describes the work undertaken by Working Group 6: Biosphere Modelling for Long Term Safety Assessments of Waste Disposal Facilities of the IAEA’s Modelling and Data for Radiological Impact Assessments (MODARIA II) programme (2016–2019). The objective of this publication is to provide updated guidance for addressing the biosphere in undertaking post-closure safety assessments for solid radioactive waste disposal, building on the original BIOMASS methodology published in 2003. The intended audience includes regulators and organizations responsible for assessing and reviewing safety assessments and assessors and modellers involved in a range of radiological studies within the nuclear industry.