How do I identify whether a reaction is SN1 or SN2?

Key factors to identify the mechanism: Substrate structure: primary alkyl halides prefer SN2 (less steric hindrance), tertiary prefer SN1 (stable carbocation), secondary can do either. Solvent: polar aprotic solvents (acetone, DMSO) favour SN2. Polar protic solvents (water, alcohols) favour SN1. Nucleophile strength: strong nucleophile favours SN2, weak nucleophile favours SN1. Reaction kinetics: SN1 is first order (rate depends only on substrate), SN2 is second order (rate depends on substrate AND nucleophile). Stereochemistry: SN1 gives racemic mixture (attack from both sides), SN2 gives inversion (Walden inversion, back-side attack only).