A tutorial introduction is provided to the relatively new subject of hybrid systems. The modelling of hybrid systems is assuming ever greater importance for systems where the combination of continuous control and with logical decision making is required. This arises in some of the most critical operating regions where systems are under start-up, shutdown or are undergoing major planned changes. There is recognition that separate independent design of these functions will reduce achievable performance and cause unpredictable behaviour in some of the most safety critical areas of operation. The introduction provided is not exhaustive but it introduces some of the main concepts and motivates the use in applications of this relatively new area ...