Blending is a technique known in polymer technology that takes advantage of theprocessibility of polymers to produce new solid materials or composites withspecific structural and physical properties, distinct from the ones of theircomponents. In thin films of polymer blends interesting morphologies are formedbecause of phase separation. For conjugated polymers, i.e. solution-processiblesemiconductors, blending also opens a way to optimize the performanceof opto-electronic devices, bringing about technological benefits. It istherefore crucial to achieve understanding of the effect film morphologyhas on the device performance, and, ultimately, to achieve control overthe phase separation in a blend, so that structures can be designed thatyield the desired device performance. Light-emitting diodes (LEDs) madeof polymer blends have shown strongly enhanced electroluminescence(EL) efficiencies, as compared to pure homopolymers. Colour conversion,white light emission, polarized light emission, emission line narrowing,and voltage-tunable colours are other effects that have been observed inblends containing light-emitting polymers. Although the enhanced ELefficiency is attributed to Förster-type energy transfer in numerous reports,the exciton dynamics behind this effect is not well understood. Here wereview the formation and morphology of thin films of conjugated polymerblends, as well as modern microscopic and spectroscopic techniques tostudy them. Furthermore, we attempt to link the film morphology to theelectronic performance of electroluminescent and photovoltaic devices anddiscuss energy and charge transfer phenomena at the interfaces. We alsoreport some new results, specifically for polyfluorene blends in LEDs.This article was originally intended for publication in Issue 42 of this volume, which was a special issue on Conjugated Polymers: Issue 42