Small RNAs (sRNAs), ~20–25 nucleotide (nt) in size, regulate various biological processes in plants through directing sequence-specific gene silencing. sRNAs are derived from either single- or double-stranded precursor RNAs. Proper levels of sRNAs are crucial for plant growth, development, genomic stability, and adaptation to abiotic and biotic stresses. Studies have identified the machineries controlling sRNA levels through biogenesis and degradation. This chapter covers recent progresses related to mechanisms governing small RNA biogenesis and degradation.