Extensive classical chemical dynamics simulations of gas-phase X - + CH 3Y→ XCH 3 + Y - S N2 nucleophilic substitution reactions are reviewed and discussed and compared with experimental measurements and predictions of theoretical models. The primary emphasis is on reactions for which X and Y are halogen atoms. Both reactions with the traditional potential energy surface (PES), which include pre- and postreaction potential energy minima and a central barrier, and reactions with nontraditional PESs are considered. These S N2 reactions exhibit important nonstatistical atomic-level dynamics. The X - + CH 3Y→ X --CH 3Y association rate constant is less than the capture model as a result of inefficient energy transfer from X -+ CH 3Y relative translation to CH 3Y rotation and vibration. There is weak coupling between the low-frequency intermolecular modes of the X --CH 3Y complex and higher frequency CH 3Y intramolecular modes, resulting in non-RRKM kinetics for X --CH 3Y unimolecular decomposition. Recrossings of the [X - CH 3 - Y] - central barrier is important. As a result of the above dynamics, the relative translational energy and temperature dependencies of the S N2 rate constants are not accurately given by statistical theory. The nonstatistical dynamics results in nonstatistical partitioning of the available energy to XCH 3 +Y - reaction products. Besides the indirect, complex forming atomic-level mechanism for the S N2 reaction, direct mechanisms promoted by X - + CH 3Y relative translational or CH 3Y vibrational excitation are possible, e.g., the roundabout mechanism.