AbstractHeterojunctions created from thin films of two dissimilar organic semiconductor materials [organic/organic′ (O/O′) heterojunctions] are an essential component of organic light emitting diode displays and lighting systems (OLEDs, PLEDs) and small molecule or polymer‐based organic photovoltaic (solar cell) technologies (OPVs). O/O′ heterojunctions are the site for exciton formation in OLEDs, and the site for exciton dissociation and photocurrent production in OPVs. Frontier orbital energy offsets in O/O′ heterojunctions establish the excess free energy controlling rates of charge recombination and formation of emissive states in OLEDs and PLEDs. These energy offsets also establish the excess free energy which controls charge separation and the short‐circuit photocurrent (JSC) in OPVs, and set the upper limit for the open‐circuit photopotential (VOC). We review here how these frontier orbital energy offsets are determined using photoemission spectroscopies, how these energies change as a function of molecular environment, and the influence of interface dipoles on these frontier orbital energies. Recent examples of heterojunctions based on small molecule materials are shown, emphasizing those heterojunctions which are of interest for photovoltaic applications. These include heterojunctions of perylenebisimide dyes with trivalent metal phthalocyanines, and heterojunctions of titanyl phthalocyanine with C60, and with pentacene. Organic solar cells comprised of donor/acceptor pairs of each of these last three materials confirm that the VOC scales with the energy offsets between the HOMO of the donor and LUMO of the acceptor ($E_{{\rm HOMO}^{\rm D} } - E_{{\rm LUMO}^{\rm A} }$).magnified image