One-step concerted mechanism: Nu:- approaches from 180° opposite the leaving group (backside attack) → pentacoordinate transition state [Nu---C---LG]‡ → LG departs as C-Nu bond forms.

Stereochemistry: Complete Walden inversion. (R) substrate → (S) product (or vice versa). Like an umbrella flipping in the wind.

Substrate reactivity: CH3X > 1° > 2° >> 3° (NEVER for 3°). Steric bulk around the electrophilic carbon blocks the nucleophile's approach. Neopentyl (1° but flanked by tert-butyl) is essentially unreactive via SN2.

Best conditions: Strong nucleophile + polar aprotic solvent (DMSO, DMF, acetone, CH3CN). Aprotic solvents don't solvate the nucleophile, keeping it "naked" and more reactive.