MicroRNAs (miRNA) are implicated in brain development and function, but the underlying mechanisms have been difficult to study in part due to the cellular heterogeneity in neural circuits. The cellular diversity of the brain necessitates studies at the level of individual neuron types. During my thesis research, I have taken two complementary approaches to study miRNA function in mouse neocortex and cerebellum. One is to analyze cell type specific miRNA expression profiles, and the other is to disrupt miRNA production in subpopulations of cortical GABAergic neurons. To establish genetic access of diverse interneuron subtypes, our laboratory has initiated the first round of a systematic effort to generate and characterize knockin Cre or CreER driver lines that target major classes and lineages of GABAergic neurons. I have contributed significantly to this effort by generating and characterizing several Cre lines that target several interneurons classes. The main component of my thesis research is the invention of a miRNA-tagging and affinity purification method, miRAP, which can be targeted to cell types through the Cre-loxP binary system in genetically engineered mice. Using this method, my miRNA profiling study of several neuron types in the neocortex and cerebellum revealed the expression of a large fraction of known miRNAs with distinct profiles in glutamatergic and GABAergic neurons, and in subtypes of GABAergic neurons. I have further detected putative novel miRNAs and miRNA editing in subset of neuron types. Our method thus will facilitate a systematic analysis of miRNA expression and regulation in specific neuron types in the context of neuronal development, physiology, plasticity, pathology and disease models, and is generally applicable to other cell types and tissues. Furthermore, I have used cell type specific knockout of Dicer to examine of role of miRNAs in interneuron development and physiology. My studies show that loss of miRNAs affects the intrinsic properties of cortical VIP neurons but not those of PV or SST neurons, while cell survival was not affected in any of the three interneuron subtypes. Thus my current results suggest that depletion of miRNAs has rather subtle effects on cortical interneurons. It is likely that miRNAs are involved in regulating the function and plasticity of interneurons in physiological and pathological conditions that are yet to be defined by future studies.