Although computers have been employed for decades to control machinery and entire laboratories, the term embedded system has received renewed attention and is about to establish itself as a discipline of its own. We therefore try to identify its characteristics and peculiarities. The most outstanding single item is the role of time. The addition of timing conditions to all other specifications of a system causes real-time programming to appear as a particularly difficult and challenging subject. We ask how the rapidly growing demands can be met, and emphasize that reliability must be guaranteed in view of the potentially disastrous consequences of failures upon the controlled equipment and its environment.