Born from clumps of Stardust spinning around a central sun, the planets of our Solar System, like most planets in the universe, began their voyage through time with a rich mixture of chemistry and energy on board. From such a cauldron, life emerged on our planet Earth early, and diversified into the great variety of living organisms that cover the surface, fill the air, and saturate the soil and oceans of our globe. Organic evolution (or “evolution” for simplicity hereafter) is the term for the historical process by which this diversification occurred. This book is about how that process could have happened on a number of different worlds both in our Solar System and beyond. In this chapter we will define life, to make sure we all have the same thing in mind, then lay out the basic requirements for life in terms of chemistry, energy, and function.