Metabolism represents a complex system characterised by a high variability in metabolites' structure, concentration and regulation ratio. Such variability is observed at different metabolic scales going from metabolites to metabolic profiles via chemical reactions and metabolic pathways, as well as under static or dynamic aspects. Variations in these components are due to apparition-disappearance, level increase-decrease and/or relative changes in weights or contributions leading to different structural, functional and evolutive states of the metabolic system. This book presents a variety of different computational approaches of the variability in metabolic systems.