This Letter explores and characterises the fundamental processes through which donor-acceptor pairs can participate in two-photon absorption. Three distinct classes of mechanism can operate: two-photon resonance energy transfer, anti-Stokes electronic Raman-induced transfer, and transfer-induced double-quantum excitation. Each is subject to the interplay of constraints associated with optical wavelength, excited state energetics and selection rules. These mechanisms may compete, or in certain cases exhibit quantum interference; some interesting thermal features are also revealed. When the two-photon response of a complex multichromophore system involves donor-acceptor pairs, monitoring the fluorescence affords a new tier of structural information. (c) 2006 Elsevier B.V. All rights reserved.