Aircraft control, production and mechatronics, active energy absorption, constructing climate control, nuclear plant process control, power generation systems, bioinformatics, electronic products, active suspensions, instant safety devices, and turbine timing are all examples of control schemes and stabilization loops in engineering. Control and stabilization may also be widely distributed in nature, such as in the homeostatic processes that allow organisms to carefully regulate internal variables like temperature, pressure, and chemical levels.

On the other hand, when one wants internal knowledge from exterior, directly available, assessments, the challenge of observer design inevitably emerges in a systematic approach. In fact, it is obvious that one cannot utilize so many more sensors as there are signals of relevance defining system activity, for financial or methodological reasons, notably these as signals might be extensive and of diverse forms. Examples of such time-varying signals that characterize the system are state variables, and unmeasured environmental factors. Thus, internal information is required for a variety of reasons, including modelling (identification), monitoring (fault detection), and driving (control) the system. To maintain a regulated system, all of these goals are demanded at the same time. As a result, the reconstruction - or observer - the problem lies at the core of a broader control issue.

This Special Issue aims to collate original research and review articles which focus on the much further systematic study of applications, many of which are described here, as the subject is still dynamic and full of outstanding challenges.

Potential topics include but are not limited to the following:

• Model-based controllers
• Stabilization of nonlinear systems
• Fault-tolerant control
• Reconstruction of states in physical nonlinear systems
• Observer design
• Linearization methods and separation principles