This chapter describes metabolic pathways that are ubiquitous among different taxa of life. This includes the use of phosphate compounds as energy carriers, the biosynthesis of carbon compounds as building blocks, enzyme-catalyzed metabolic reactions, and the use of nucleotide bases to store information. These vital processes define primary metabolism, while the secondary metabolism, which varies more significantly from species to species, fulfills more specific and often ecological functions. Secondary metabolites are characterized by their taxonomic specificity, their structural diversity, and their often not clearly identifiable biological function. The biosynthesis of secondary metabolites branches off from intermediates of primary metabolism. Hence, a clear distinction between primary and secondary metabolism is often difficult to make. The representation of metabolic pathways, as illustrated by databases like KEGG, MetaCyc, and others reveals the complexity of biochemistry and shows the links between the individual biosynthesis pathways. This chapter aims to introduce basic metabolic pathways and analyze their relationships in order to convey biochemistry as a systematic science, the key approach of this book.