MicroRNAs (miRNAs) are an important class of endogenous small silencing RNAs in both plants and animals. They regulate the expression of a wide range of target genes that are involved in many important biological processes. Biogenesis of plant miRNAs requires a distinct set of proteins, including members that belong to several highly conserved RNA silencing protein families. The framework for miRNA biogenesis in plants was revealed through genetic and biochemical analyses using mutants that are defective in miRNA accumulation. These general miRNA-deficient mutants constitute a set of invaluable genetic resources for the plant miRNA research community. They could be utilized to experimentally validate the candidate miRNAs that are either predicted by a computational program or recovered from a small RNA deep sequencing effort which is becoming a more affordable and widely used approach for small RNA discovery. Starting with a brief introduction on multiple small RNA pathways in plants, this chapter provides basic experimental procedures for the examination of miRNA accumulation from wild type plants and various mutant lines in Arabidopsis.