Proteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues. The sequence of amino acids in a protein is defined by the sequence of a gene, which is encoded in the genetic code. In general, the genetic code specifies 20 standard amino acids; however, in certain organisms the genetic code can include selenocysteine and in certain archaea pyrrolysine.

It is estimated that the human body may contain over two million proteins, coded for by only 20,000 - 25,000 genes. The total number of proteins found in terran biological organisms is likely to exceed ten million, but nobody knows for sure. We have data on just over a million proteins, taken mainly from information found in the 100 genomes which have been fully sequenced as of 2007.

The longest known protein, titin, also known as connectin, contains 26,926 amino acids. Titin is found in muscle and contributes to its passive stiffness. Because the 20 amino acids can be connected up in arbitrary sequences, the total space of possible proteins is exponential, with a value of approximately 2050,000, a tremendous number. Within this space of proteins are probably cures for every disease or ailment, but locating these in such a vast space is a profound computational and theoretical challenge.

The word protein comes from the Greek prota, meaning "of primary importance". This is a suitable name, as the central importance of proteins in the human body can not be overestimated. All biological organisms can be seen fundamentally as protein structures filled with water and sometimes supported by mineralized tissues called bone. For almost every protein there is another protein that can break it down. Proteins sometimes coalesce into mutually cooperative units called complexes, which perform useful biological functions. Every section of useful genetic information, found in the RNA or DNA, codes for a corresponding protein which goes on to fulfill a useful biological role.

The field that analyzes proteins in general and aims to exhaustively characterize all proteins in the human body is called proteomics. Many see the next logical step after the completion of the Human Genome Project to be the initiation of a Human Proteome Project. The Human Proteome Organization was founded to pursue this goal.