100 years of Human Chorionic Gonadotropin: Reviews and New Perspectives is a collection of articles written by some of the world’s leading experts on the pregnancy hormone and cancer marker hCG. In 2019 it is difficult to ignore the effect that our understanding of hCG has had on the lives of millions of people worldwide. The hCG immunoassay, in one form or another, is now one of the most common medical tests conducted and is often the first indication that a mother-to-be is pregnant. Not only a marker of pregnancy, hCG is utilized in the diagnosis and monitoring in oncology and presents a potential target for novel cancer therapeutics. 100 years ago, in 1919, Hirose was demonstrating gonadotropic functions which resulted from a chorionic factor. Over the last century this factor has become defined as hCG and more recently explored as not one molecule but a group of molecules with variable structure and variable functions in both pregnancy and cancer. hCG is a multi-faceted molecule that has clinical and therapeutic implications but can be a challenging topic for researchers and physicians alike. This text covers the different structures and functions of hCG exploring the genes and evolution of the molecule, the different protein and glycosylation structures which can exist and their effect on structure, detection and quantification. 100 Years of hCG is not an attempt to recount the history of every publication on hCG, but rather a collection of reviews and new perspectives by "hCG-ologists", the term used by Hussa to describe biochemists working on HCG in the first book written on the topic over 30 years ago. Some of the authors have been around a while, some not so long, but others are just beginning their journey with a most beguiling molecule.