Biopharmaceuticals have revolutionized the treatment and management of many diseases. The advent of recombinant erythropoietins has greatly benefited patients with anemia related to chronic kidney disease and cancer, virtually eliminating the need for blood transfusions. Currently, the patents for many biopharmaceutical molecules have expired or are approaching expiration and a number of biosimilars manufacturers are aiming to claim part of the market share. Unlike the situation for synthetic "small molecule" drugs, identical copies of far more complex biopharmaceuticals cannot be produced. A biopharmaceutical can be 100 to 1000 times larger than a synthetic chemical drug, with extremely complex three-dimensional structure and biological functions which are often not completely understood. Due to their nature and complexity, these fascinating therapeutic molecules are products of highly controlled biological processes. This review takes a look at how biosimilars are fundamentally different from their originator products by examining the biopharmaceutical production process and how it can influence the structure and function of the final drug product.