Significant improvements have been made during the last 20 years in therapy of renal diseases including the broadening of treatment options. Gene therapy is a potential modality for many renal diseases for which we are yet unable to offer specific treatment. Here, we introduce RNA interference (RNAi), one type of posttranscriptional gene silencing, as a novel gene therapeutic possibility and describe the mechanism and kinetics of action. We highlight the correlation between structure and efficacy of small interfering and short hairpin RNAs that are the most often used small RNAs possessing RNAi activity. Delivery is the biggest obstacle for RNAi-based gene therapy. Although hydrodynamic treatment is effective in animals, it cannot be used in human therapy. Possibilities to achieve site-specific and effective delivery are listed. Side effects of RNAi and potential solutions are also summarized. Besides the above-described world of small RNAs, we draw attention to the yet unrevealed function of human microRNAs that are localized mainly in the noncoding regions of the genome, are highly conserved among animals and possess important regulatory functions. Although there are many unanswered questions and problems to face in this new field of gene therapy, we summarize a number of experiments targeting renal diseases with the aid of RNAi. High specificity of short interfering RNAs and short hairpin RNAs raise hope for treating renal diseases.