RNAi is RNA interference by short RNAs. It influences gene-expression by down-regulation of mRNAs, typically by complementarity to the 3 UTR (untranslated region) of the mRNA. microRNAs (miRNAs) are short RNAs acting as natural RNAi. miRNAs mediate down-regulation of many mRNAs from developmental genes and transcription factor genes. Natural examples for this additional level of post-transcriptional control are increasing. Suitable computer-based search strategies for new miRNA candidates include precursor folding as well as different compositional search strategies. Example programs for this are presented. New own and other data are provided for an overview on such strategies. A strategy feasible in plants for miRNA target identification is direct base pairing of miRNAs to potential mRNA target 3 UTRs. Correct identification in animals usually requires comparative genomics and conserved UTR regions pairing to conserved miRNA substructures. A number of example programs and target examples for these tasks are examined. Finally, strategies and programs for artificial gene silencing by designed RNAi are explained.