The chapter discusses the development and experimental advancements in the field of electron transfer reactions since the late 1940s. Initially, the focus was on "isotopic exchange reactions" and later "cross reactions," driven by the availability of radioactive isotopes and new instrumentation that allowed the study of rapid chemical reactions. The simplicity of self-exchange reactions, where products are identical to reactants and no chemical bonds are broken or formed, simplified the study of electron transfer. New instrumentation, such as the stopped-flow apparatus, enabled the study of fast electron transfer reactions in solution. The introduction of lasers and nanometer-sized electrodes has further advanced the field, allowing studies in picosecond and subpicosecond time regimes. The interaction between theory and experiment has been extensive, with theoretical developments like the RRKM theory and experimental advancements in electrochemistry and biology contributing to the field. The chapter also highlights the importance of theoretical predictions, such as the cross-relation and the inverted region effect, which have been experimentally verified. The author, Rudolph A. Marcus, describes his own involvement in the field, starting with early experiments on isotopic exchange reactions and evolving into the development of electron transfer theory. The theory, which accounts for the role of fluctuations and solvent dynamics, has been applied to various systems, including biological processes like photosynthesis. The chapter concludes by discussing the broader applications and extensions of electron transfer theory in chemistry and biology.The chapter discusses the development and experimental advancements in the field of electron transfer reactions since the late 1940s. Initially, the focus was on "isotopic exchange reactions" and later "cross reactions," driven by the availability of radioactive isotopes and new instrumentation that allowed the study of rapid chemical reactions. The simplicity of self-exchange reactions, where products are identical to reactants and no chemical bonds are broken or formed, simplified the study of electron transfer. New instrumentation, such as the stopped-flow apparatus, enabled the study of fast electron transfer reactions in solution. The introduction of lasers and nanometer-sized electrodes has further advanced the field, allowing studies in picosecond and subpicosecond time regimes. The interaction between theory and experiment has been extensive, with theoretical developments like the RRKM theory and experimental advancements in electrochemistry and biology contributing to the field. The chapter also highlights the importance of theoretical predictions, such as the cross-relation and the inverted region effect, which have been experimentally verified. The author, Rudolph A. Marcus, describes his own involvement in the field, starting with early experiments on isotopic exchange reactions and evolving into the development of electron transfer theory. The theory, which accounts for the role of fluctuations and solvent dynamics, has been applied to various systems, including biological processes like photosynthesis. The chapter concludes by discussing the broader applications and extensions of electron transfer theory in chemistry and biology.