Polymorphic electronics is a new domain of electronics comprising digital circuits, which are able to perform more than one function. Polymorphic circuits typically have one stable structure for all functions and actually performed function depends on an environment. All functions of the circuit are designed intentionally. Polymorphic circuits are very area efficient in comparison to conventional multi-function circuits. Change of the circuit function comes immediately (with no delay) and sensitivity to the environment is embedded to the circuit. Typically, power supply voltage, temperature or a special signal serves as the environment which determines the function of the circuit, i.e. physical quantities that affect behaviour and parameters of semiconductors. But the concept of polymorphic electronics is generalised in this thesis, although todays applications are based on unipolar semiconductor transistors. There are three basic problems of polymorphic electronics: design methods for polymorphic circuits, lack of suitable polymorphic components (gates) and identification of appropriate application areas. In this thesis, several possible solutions of all three main problems of polymorphic electronics are described. The thesis contains description and evaluation of almost all existing polymorphic gates. Also known methods of polymorphic circuits synthesis are introduced and discussed. Lot of polymorphic circuits were designed using evolutionary design methods, especially using Cartesian Genetic Programming. But also non-evolutionary (conventional) design methods were proposed. A range of applications is described in the thesis. Majority of them were implemented and verified by physical realisation. Generally, polymorphic circuits can be used in applications that assume adaptation of the circuit to variable environment or smart and fast reconfiguration. Such behaviour is useful for circuits that must adapt itself to unfriendly environment e.g. by restriction of power consumption or heat dissipation with preservation of necessary basic functionality. Polymorphic electronics is profitable also in applications that are basically mono-functional, but need some additional feature. The additional feature may be used rarely, but due to typical area effectiveness of polymorphic circuits, the additional feature is keenly priced. This is attractive e.g. for embedded diagnostics, security applications etc. Principles of polymorphic electronics are useful for both combinational and sequential circuits. It is shown in the thesis how to utilise polymorphic electronics for design of safe adaptive circuit controllers. For real experiments, the first reconfigurable polymorphic chip in the world was designed and manufactured. The text of the thesis, and namely definitions and assertions stated in the thesis done by the author, reflects also practical experiences acquired during years of work with polymorphic electronics.