Protein post-translational modification (PTM) occurs following their biosynthesis and is a key cellular event that defines their ultimate functional properties. It is an important control mechanism for display of biological functions of proteins often in a profound manner. It may switch on or off a protein's function. Several studies have been conducted to understand their mechanisms, physiological pathways and functional properties. PTMs have been shown to alter structural, conformational and physicochemical properties of proteins. So far a variety of protein modifications have been detected in physiological systems. These involve covalent modifications of amino acids via their side chains, backbone peptide bonds and terminal moieties. Following PTM, proteins may become (a) pathologically toxic, (b) biologically active or inactive, (c) more or less susceptible to proteolytic processing, (d) increasingly/decreasingly bound to its partner protein/s, or (e) modified with altered protease activities. These changes may affect pathways linked to cell signaling/transduction, trafficking, storing, expression, binding and/or affinity. Any of these events may be linked to metabolic, growth and/or chronic dysfunctions with serious health consequences that may include cancer, cardiovascular disease, stroke, viral/bacterial/parasite infections, inflammation, thrombosis, diabetes; central nervous system related conditions. Some of the modifications are more prevalent physiologically and widely studied. However, in recent years additional PTMs have been described that are less common. These include glypiation, neddylayion, siderophorylation, sumoylation, AMPylation, Cholesteroylation and others which are also important. This manuscript provides a comprehensive review of these rare and unconventional types of protein modifications and their functional implications to health, metabolism and disease conditions.