Scheme 4.1, two intersecting parabolas, represents the Marcus theory of electron transfer. It is, arguably, as recognisable to chemists, physicists and biologists as the Volkswagen logo is to the general public. This powerful two-state theory has spawned literally thousands of detailed computations and simulations, including some from our own groups. More importantly, it has provided the conceptual framework for the interpretation of tens, if not hundreds, of thousands of electron transfer experiments. Rudy Marcus is a true giant of physical chemistry, on whose shoulders stand a legion of adopters and adapters; a scientific hero of the first magnitude. His Nobel Prize was more than 20 years ago and he is still going strong at the age of 92, publishing at a rate and an impact that would shame researchers 40 years his junior, or more! In terms of bibliometrics, he has published over 450 papers that have been cited nearly 50 000 times. His most cited paper has more than 6000 citations and he has six papers with more than 1000. His h-index is a staggering 94 (94 papers with more than 94 citations). We respectfully dedicate this review chapter to Professor Marcus.