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RNA-directed DNA methylation (RdDM) is a plant-specific de novo DNA methylation pathway, which is required for the silencing of transposable elements and transgenes. In the past decade, the main components of the RdDM pathway were identified, and the functions of these components were well studied. In this chapter, we summarize our understanding of the RdDM pathway and especially focus on recent works of the field. In this pathway, Pol IV-dependent small interfering RNAs (siRNAs) are assembled onto the Argonaute protein AGO4, and base-pair with Pol V-dependent scaffold noncoding RNAs, thereby targeting the RdDM effecter complex to their homologous genomic loci. By associating with AGO4, the de novo DNA methyltransferase DRM2 is guided to the loci and catalyze DNA methylation.