This chapter delves into the structure and biosynthesis of various natural substances, which are composed of diverse building blocks. These building blocks are classified according to Dewick (2009) and include C1 to C5N. They are formed through various enzymatic reactions, with many enzymes exhibiting some substrate promiscuity and therefore being able to convert multiple substrates. The biosynthesis of natural substances takes place in living cells and is based on enzymes that catalyze biochemical reactions in both primary and secondary metabolism. Understanding how these enzymatic reactions relate to each other is crucial to comprehend the biosynthesis of natural substances. The chapter discusses some of these enzymatic reactions and highlights the importance of substrate promiscuity. It provides an overview of the structure and biosynthesis of natural substances and lays the foundation for further discussions on enzyme kinetics and enzymatic catalysis.