Since some years, activation of RNA interference (RNAi) has been developed into a widely used and powerful biological tool to functionally annotate genomes. Several variants of small regulatory RNAs can trigger this evolutionary highly conserved cellular pathway leading to specific hybridisation to and subsequent degradation or translational repression of target mRNAs. These regulatory RNAs include synthetic double-stranded small interfering RNAs (siRNAs), pol III transcribed small hairpin RNAs (shRNAs), or endogenous or artificial micro RNAs (miRNAs) which are expressed from pol II promoters as primary pri-miRNA transcripts subsequently processed into mature miRNAs in a regulated multi-step process. Depending on the mode of activation RNA-processing and efficacy as well as kinetics of RNAi may differ from transient effects to long-lasting gene silencing. However, one of the major challenges for in vitro and in vivo application of RNAi still remains the efficient delivery of suitable RNAi-triggers to target cells. This review highlights the mechanism of RNAi and its activation, the use of plasmid-based and retro-/ lentiviral vector-based systems to mediate long-term RNAi, kinetic aspects of RNAi and their impact on selection of cell populations with modified gene expression.