HYPAS – A Modular Structured Model Design, Simulation, and Control Programming Environment. The importance of Computer Aided Design, Modeling, and Simulation (CADMS) for automatic control has became prominent in recent years. During the past 25 years, many CADMS tools have been designed in various domains. Nowadays, the user is interested in quick design of plants, regardless their complexity, without being forced to write differential equations or to manipulate complicated formulae accounting non linearity. This is also required in hydraulics and pneumatics, where automation proves lately an increasing development. For these 2 fields, an approach based on the concept of “Modular Structured (Mathematical) Modeling” has been introduced and developed in the last 15 years. This effort resulted in a powerful software product, HYPAS, devoted to the interactive design and automatic generating of mathematical models for hydraulic and pneumatic drive installations, as well as to simulation of their behavior. The article is aiming to introduce the philosophy and the main features of HYPAS, as main tool in CADMS of hydraulic and pneumatic installations.