RNA interference (RNAi) has emerged as an important tool in science and in medicine. Small-interfering RNAs (siRNAs) can be used to knockdown gene expression of specific mRNAs. In practice, a number of factors influence whether an siRNA sequence will elicit RNAi and knockdown target gene expression. One factor that significantly influences the efficiency of an siRNA is the effect of RNA secondary structure. Self-structure in either the siRNA sequence or the target mRNA at the binding site may prevent gene silencing. This chapter provides protocols for using the OligoWalk software package to design efficient siRNAs. OligoWalk considers the effect of target and guide strand self-structures and also local sequence features in siRNA design. OligoWalk can be run either locally by compiling the software or through a convenient web interface. OligoWalk is freely available at http://rna.urmc.rochester.edu/cgi-bin/server_exe/oligowalk/oligowalk_form.cgi .