This chapter examines the thermodynamic arguments and discusses algorithmic information. It looks at the ideas of Dr Rolf Landauer, Edward Fredkin, Tommaso Toffoli, Norman Margolus, and Charles Bennett, concerning the reversibility of computation and what that could mean physically. The chapter also examines how systems could be universal and in what sense they could be models for the physical world. It also discusses what was then known about the quantum mechanics of computation. Dr Richard Hughes explained his work on the computational difficulty of simulating a quantum-mechanical system with a classical computer. He also gave a few lectures in which he developed a quantum-mechanical state machine, composed of a programmed logic array built up from operators, and a state register described in terms of spin states. The chapter investigates the limitations of size for circuits based on lithography and methods such as ion implantation and diffusion.