During the first years of The Journal of Physical Chemistry, chemists were just beginning to understand chemical reactions in gases as sequences of elementary reaction steps. Basic models of reaction dynamics were developed in succeeding decades which provided powerful qualitative insight into reaction mechanisms and rate constants. In recent decades, the development of molecular beam and laser technologies has allowed us to look into the transition state region at the dynamics of chemical bond formation and breaking. State-selective preparation of reactants and state-resolved detection of products with velocity and angle resolution permit exacting quantitative test of dynamical theories. Thanks to the ability to calculate accurate nb initio potential energy surfaces and solve Schroedinger’s equation for reactive scattering dynamics, the simplest reactions are now understood quantitatively at the most fundamental level possible. The concepts and models thus developed, along with ever more powerful experimental and computational tools, are rapidly being applied to more complex reactions.