To investigate the role of Dicer and microRNAs in the mammalian CNS, we used mice in which the second RNase III domain of Dicer was conditionally floxed. Conditional Dicer mice were bred with mice expressing an alpha-calmodulin kinase II Cre to selectively inactivate Dicer in excitatory forebrain neurons in vivo. Inactivation of Dicer results in an array of phenotypes including microcephaly, reduced dendritic branch elaboration, and large increases in dendritic spine length with no concomitant change in spine density. Microcephaly is likely caused by a 5.5-fold increase in early postnatal apoptosis in these animals as determined by active caspase-3 and TUNEL (terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling) staining in the cortex. Loss of Dicer function had no measurable effect on cortical lamination as determined by in situ hybridization, suggesting that microcephaly is not caused by defects in neuronal migration. Together, these results illustrate the in vivo significance of Dicer and miRNAs in the mammalian CNS and provide additional support for previous in vitro studies indicating that misregulation of this pathway may result in gross abnormalities in cell number and function that may contribute to a variety of neurological disorders.