This chapter discusses the requirements for a world to be deemed habitable at a given moment in time (instantaneous habitability), with an emphasis on the availability of energy sources and suitable physicochemical conditions. After a brief exposition of some concepts in thermodynamics, the significance of the molecule ATP (the ‘energy currency’ of the cell) and how it is synthesised in the cell by harnessing chemical gradients is described. The two major sources of energy used by life on Earth (chemical and light energy), and the various possible pathways for utilizing such forms of energy are sketched, most notably photosynthesis and methanogenesis. This is followed by delineating the diverse array of extremophiles that inhabit myriad niches on Earth that would be considered harsh for most life. The mechanisms that permit them to survive the likes of high/low temperatures, pressures, salinity, and radiation doses are reviewed.