The main goal of a sensing device is to provide reliable, precise, up-to-date and concise information about a process variable, parameter or index, to be used for different monitoring or control purposes. Other than using the basic transducer device, it is also possible to convert a variable into another one which is easier to handle. Modern computer-based systems allow for more versatile sensors providing such information without the need for extra hardware. The use of AI techniques also allows for some form of intelligence in performing this task. Nevertheless, the use of model based sensors introduces unavoidable performance penalties. Also, various constraints apply to their design. In this paper, sensing goals and activities are reviewed, as well as different approaches to achieve them. A basic analysis of their properties for real-time operation and their limitations is presented and some applications are discussed.