RNA interference (RNAi) is a recently discovered genetic immune system with widespread therapeutic and genomic applications. In this paper, we address the problem of selecting an efficient set of initiator molecules (siRNAs) for RNAi-based gene family knockdown experiments. Our goal is to select a minimal set of siRNAs that (a) cover a targeted gene family or a specified subset of it, (b) do not cover any untargeted genes, and (c) are individually highly effective at inducing knockdown. We show that the problem of minimizing the number of siRNAs required to knock down a family of genes is NP-Hard via a reduction to the set cover problem. We also give a formal statement of a generalization of the basic problem that incorporates additional biological constraints and optimality criteria. We modify the classical branch-and-bound algorithm to include some of these biological criteria. We find that, in many typical cases, these constraints reduce the search space enough that we are able to compute exact minimal siRNA covers within reasonable time. For larger cases, we propose a probabilistic greedy algorithm for finding minimal siRNA covers efficiently. Our computational results on real biological data show that the probabilistic greedy algorithm produces siRNA covers as good as the branch-andbound algorithm in most cases. Both algorithms return minimal siRNA covers with high predicted probability that the selected siRNAs will be effective at inducing knockdown. We also examine the role of "off-target" interactions -the constraint of avoiding covering untargeted genes can, in some cases, substantially increase the complexity of the resulting solution. Overall, however, we find that in many common cases, our approach significantly reduces the number of siRNAs required in gene family knockdown experiments, as compared to knocking down genes independently.