This document describes the technical details, construction and operation of the fully revised multimedia fate model Simplebox 3.0. Former versions of Simplebox have been used for prediction of intermedia concentration ratios in order to harmonisation of environmental quality objectives. Also Simplebox is (partly) built in the "European Union System for the Evaluation of Substances" (EUSES) As an illustration a fully worked out example computation is admitted Simplebox 3.0 is a nested multi-media fate model of the “Mackay type”. The environment is modelled as a set of forty well-mixed, homogeneous compartments (air, fresh water and marine water compartments, sediment, soil and vegetation compartments) on a local., regional, continental and global scale. The global scale represents the Northern Hemisphere and contains a moderate, arctic and tropical zone. The model needs substance properties, emission rates and transformation rate constants as input and computes steady-state concentrations for all boxes as output. Simplebox is a generic model in the sense that it can be customised to represent specific environmental situations. With respect to Simplebox 2.0, several changes have been made. The main modifications are the addition of a local scale nested in the regional scale and refinement of the seawater currents between the regional, continental and moderate zone of the global scale. All parameters are renamed and the lay out of the model has been changed.