Requirements: At least one compound must have alpha-H; the other provides the electrophilic C=O.

Base-catalyzed mechanism:

1. NaOH abstracts alpha-H → enolate anion (resonance-stabilized)
2. Enolate (nucleophilic at C) attacks another molecule's C=O
3. Alkoxide protonated → beta-hydroxy aldehyde/ketone (aldol)
4. Heating → dehydration → alpha,beta-unsaturated carbonyl (conjugated, more stable)

Crossed aldol (mixed aldol): One component has no alpha-H (benzaldehyde, formaldehyde) → it can only act as electrophile. The other provides the enolate. This avoids multiple product formation.

Claisen-Schmidt condensation: Crossed aldol between an aromatic aldehyde (no alpha-H) and a ketone with alpha-H. Example: PhCHO + CH3COCH3 → PhCH=CHCOCH3 (benzalacetone).