RNA-induced silencing complex is the cytoplasmic effector machine of the microRNA (miRNA) pathway and contains a single-stranded miRNA guiding it to its target mRNAs. The biogenesis of mature miRNAs is a regulatory process achieved by complex machinery composed of few protein components, among which the ribonuclease III Dicer plays a central role. Dicer is essential for miRNA maturation and catalyzes one of the rate-limiting steps of miRNA production. In this chapter, we describe a protocol to study the catalytic activity of Dicer in cell extracts by measuring their ability to process precursor RNA (pre-miRNAs) into functional mature miRNAs. Impairment of the miRNA biogenesis machinery due to loss-of-function mutations in effectors of the pathway such as Dicer has been demonstrated before. Dicing assay can be used in cancer to assess Dicer enzymatic activity compared with healthy controls. Therefore, this experimental approach is likely to be useful to researchers investigating the main steps of miRNA biogenesis and function in human health and diseases.