MicroRNAs (miRNAs) are a class of small RNAs that are approximately 22 nucleotides in length. Hundreds of miRNA genes are encoded in the animal genome, and each miRNA potentially regulates tens to hundreds of protein-coding transcripts post-transcriptionally. Experimental and bioinformatic approaches have shown widespread regulatory roles for miRNAs in metazoa including roles in cellular homeostasis and human diseases. Since the discoveries of let-7 and lin-4 miRNAs as regulators of developmental timing in Caenorhabditis elegans, functions of miRNAs in the context of animal development have been studied in many model organisms. Although miRNAs are essential to achieve complex developmental processes, the vast majority of animal miRNA functions have yet to be determined. The identification of miRNA-target interactions and the interpretation of their biological significance are often difficult due to the divergent functions of miRNAs in intricate gene regulatory networks. This review summarizes our current knowledge on miRNA functions in vertebrate development by focusing on the progress made in the vertebrate model organism zebrafish (Danio rerio). Studies of miRNA functions in this small teleost highlight several common principles underlying the functions of animal miRNAs.