Despite its fundamental importance, the molecular mechanisms that regulate plant embryogenesis remain largely uncharacterized. Studying the embryonic functions of microRNAs (miRNAs) will not only yield significant insights into the mechanistic basis of plant embryo development, but will also contribute to our general understanding of how small regulatory RNAs influence cellular differentiation. More specifically, miRNAs are a class of ~21 nucleotide regulatory RNAs that are essential for both embryonic pattern formation and developmental timing in Arabidopsis thaliana. Because many plant miRNAs specifically repress transcripts encoding transcription factors, miRNAs likely have a large influence on the gene regulatory networks that control plant embryogenesis. We are investigating the functions of embryonic miRNAs with a combination of classic and modern molecular embryology techniques.