Post-translational modification (PTM) of proteins is an essential means of allowing rapid regulation of protein function in all organisms. This process, which is typically reversible and extremely dynamic, allows cells to respond rapidly to environmental factors, be that e.g. growth factors, stressors, or contact adhesion with other cells. Consequently, dys-regulation of PTM status is often a cause of disease, including e.g. cancer, diabetes and auto-immune disorders. To understand how different types and sites of protein PTM contribute both to fundamental and disease biology, it is essential to catalogue these PTMs in a high-throughput manner across the whole protein complement, and quantify how they are regulated under different conditions. However, the vast majority of PTM discovery studies focus on specific types of modifications, particularly phosphorylation of serine, threonine and tyrosine residues. Recent technology development in our group has revealed that many other amino acids are also phosphorylated on human proteins and suggests that there is extensive interplay between different types of PTM on the same residues.