Evolvable hardware (EH) is a field focusing on the use of evolutionary algorithms (EA) to create specialized electronics without manual engineering. It brings together reconfigurable hardware, evolutionary computation, fault tolerance and autonomous systems. Evolvable hardware refers to hardware that can change its architecture and behavior dynamically and autonomously by interacting with its environment. Introduction In its most fundamental form an evolutionary algorithm manipulates a population of individuals where each individual describes how to construct a candidate circuit. Each circuit is assigned a fitness, which indicates how well a candidate circuit satisfies the design specification. The evolutionary algorithm uses stochastic operators to evolve new circuit configurations from existing ones. Done properly, over time the evolutionary algorithm will evolve a circuit configuration that exhibits desirable behavior. Each candidate circuit can either be simulated or physically implemented in a reconfigurable device. Typical reconfigurable devices are field-programmable gate arrays (for digital designs) or field-programmable analog arrays (for analog designs). At a lower level of abstraction are the field-programmable transistor arrays that can implement either digital or analog designs. The concept was pioneered by Adrian Thompson at the University of Sussex, England, who in 1996 used an FPGA to evolve a tone discriminator that used fewer than 40 programmable logic gates, and had no clock signal. This is a remarkably small design for such a device, and relied on exploiting peculiarities of the hardware that engineers normally avoid. For example, one group of gates has no logical connection to the rest of the circuit, yet is crucial to its function. Motivation In many cases, conventional design methods (formulas, etc.) can be used to design a circuit. But in other cases, the design specification doesn't provide sufficient information to permit using conventional design methods. For example, the specification may only state desired behavior of the target hardware. In other cases, an existing circuit must adapt—i.e., modify its configuration—to compensate for faults or perhaps a changing operational environment. For instance, deep-space probes may encounter sudden high radiation environments, which alter a circuit's performance; the circuit must self-adapt to restore as much of the original behavior as possible. Finding the fitness of an evolved circuit The fitness of an evolved circuit is a measure of how well the circuit matches the design specification. Fitness in evolvable hardware problems is determined via two methods: extrinsic evolution: all circuits are simulated to see how they perform, intrinsic evolution: physical tests