Small interfering RNAs (siRNAs) are widely used to study gene function owing to the ease with which they silence target genes, and there is considerable interest in their potential for therapeutic applications. In a remarkably short time since their discovery, siRNAs have entered human clinical trials in various disease areas. However, rapid acceptance of the use of siRNAs has been accompanied by recognition of several hurdles for the technology, including a lack of specificity. Off-target activity can complicate the interpretation of phenotypic effects in gene-silencing experiments and can potentially lead to unwanted toxicities. Here, we describe the types of off-target effects of siRNAs and methods to mitigate them, to help enable effective application of this exciting technology.