Biological functions require protein and the protein makeup of a cell determines its behaviour and identity. Proteins, therefore, are the most abundant molecules in the body except for water. The approximately 20,000 protein coding genes in the human genome can, by alternative splicing, multiple translation starts, and post-translational modifications, produce over 1,000,000 different proteins, collectively called ‘the proteome’. It is the size of the proteome and not the genome that defines the complexity of an organism. ‘Proteins’ describes the composition and structure of proteins and how they are studied. What information is required in order to understand how proteins work and what happens when this function is impaired in disease?