Accounting for interactions is an important feature in ecological modelling when dealing with complex systems. ALWAYS is an object-oriented model which describes the biophysical functioning of a silvopastoral system at a plot and daily scale. The main processes linking the five components of the system (climate, tree, sward, soil and animal) are described. We particularly point out how the interactions between the different components of the system and resources sharing have been modeled. The main biophysical equations are developed (tree and sward growth, water cycle, transmitted PAR). They are coded in separate dynamic link libraries to enable easy updates, and are linked together through a main core programme. ALWAYS offers three ways of running it: (i) interactively by running the simulation year by year and checking results at the end of each year using graphs and output files; (ii) automatically by describing the management over the full tree rotation and (iii) in a sensitivity analysis mode to test parameters from the yearly or rotation outputs. Examples of output files, written in an ASCII format, are provided. Such a model is the first step towards a more effective decision support tool.