Silvopastoral systems are complex biological systems which combine long-term production (timber and firewood) and annual production (forage, meat and fibre). These systems involve interactions between the different components in terms of space occupation (vertical and horizontal stratification - above and below ground) and resource allocation (light, water nutrients). The structure of Object Oriented modelling offers an efficient method to deal with such complex biological processes as it allows each component of the system to be described in an adequate way (state variables and evolution processes) and to use the relationships between objects to account for the interactions within the system. Always is a model designed to make biophysical assessment of a plot-based silvopastoral system by using the power of an object oriented design. The flexibility of Windows programming enables different usergroups to be targetted - from scientists to agriculture advisors - by means of graphic user interfaces adapted to their needs regarding the functioning of the different processes. The present version of the software (Always 1.0) is mainly a research tool, but it is hoped that future improvements will lead to more applied uses. Each component of the system is coded in a separate dynamic link library, using the most pertinent published models from the literature to describe the growth processes (tree and sward) and the different budgets (nitrogen, carbon and water). Specific experiments have provided data for parameter estimation. All libraries are linked through a main core programme. Management practices are either entered on an annual basis (interactive process) or pre-recorded in a management scenario file (automatic process). Abundant outputs (graphs and ASCII files) help analyse the different processes, the interactions between different components and the results of various management practices. A sensitivity analysis process is also available for testing the different parameters entered in the model.