Gene-directed therapy with small interfering RNA (siRNA) has a tremendous potential and will undoubtedly occupy one of the leading positions among other therapeutic methods in the future. The lack of efficient and targeted delivery vectors delays a successful implementation of this method in medicine. To develop such systems, one needs a comprehensive insight into the interactions of siRNAs, its delivery systems, and an organism. The review covers the properties of therapeutic siRNAs and nonviral systems for their delivery.