This document describes the technical details of the multimedia fate model SimpleBox, version 1.0 (930801). SimpleBox is a multimedia box model of what is commonly referred to as a "Mackay-type" model ; it assumes spatially homogeneous environmental compartments (air, water, suspended matter, aquatic organisms, sediment, three soil compartments). The model takes emission rates and rate constants for transport and transformation of micropollutants as input and computes concentrations in the environment as output. In its default setting, the SimpleBox computation represents the behaviour of micropollutants in an open system, resembling the Netherlands. The present version of the model was prepared to serve as a means to support regulatory decision making in the Netherlands. This report is meant to provide the technical documentation that is necessary to serve these purposes. The model is distributed on diskette, along with this document.