MicroRNAs (miRNAs) are important post-transcriptional regulators of their target genes in plants and animals. miRNAs are usually 20-24 nucleotides long. Despite their unusually small sizes, the evolutionary history of miRNA gene families seems to be similar to their protein-coding counterparts. In contrast to the small but abundant miRNA families in the animal genomes, plants have fewer but larger miRNA gene families. Members of plant miRNA gene families are often highly similar, suggesting recent expansion via tandem gene duplication and segmental duplication events. Although many miRNA genes are conserved across plant species, the same gene family varies significantly in size and genomic organization in different species, which may cause dosage effects and spatial and temporal differences in target gene regulations. In this review, we summarize the current progress in understanding the evolution of plant miRNA gene families.