A conjecture of Manin predicts the distribution of rational points on Fano varieties. We provide a framework for proofs of Manin's conjecture for del Pezzo surfaces over imaginary quadratic fields, using universal torsors. Some of our tools are formulated over arbitrary number fields. As an application, we prove Manin's conjecture over imaginary quadratic fields K for the quartic del Pezzo surface S of singularity type A 3 with five lines given in P 4 K by the equationsQ, and [FMT89, Appendix] and [PT01] for computational evidence in degree 3 over Q).The most important technique is the use of universal torsors, which were invented by Colliot-Thélène and Sansuc (see [CTS87], for example) and first applied to Manin's conjecture by Salberger (see [Pey98, Sal98]). The testing ground was a new proof in the case of split toric varieties over Q (see [Sal98]). 1 After submission of the present article, Loughran [Lou13] showed how to derive this over arbitrary number fields from the work of Skinner [Ski97].