The importance of computer aided design, modeling, and simulation in the field of control systems has been prominent in recent years. Hydraulics and pneumatics are automation fields with increasing development. The user of these installations is interested to design an plant as quick and as well as possible, if it is possible without to be obliged to write down own differential equations. The author developed an own approach and finally a software, called HYPAS, destined to aid the engineer to easy mathematical describe/model and to simulate its plants, without the user must write down any differential equations and to manipulate formulas and nonlinearities. HYPAS is a software tool that allows the user, by just moving of assigned icons to the functional elements, to interactively design and automatically generate mathematical models of hydraulic and pneumatic drive installations, as well as to simulate them. The aim of the paper is to introduce HYPAS with its philosophy and approaches as work medium and to present some of its tools.