As information flows through a cortical circuit, layers 2/3 provide an important area for signal processing and integration, and inhibitory interneurons are crucial to this function. Diversification of inhibitory interneurons provides the means for diversification of function. Inhibitory interneurons have been classified based on morphology, electrophysiology, and molecular markers, and each classification provides additional insight into how an interneuron type could shape information flow in the cortical circuit. Parvalbumin (PV) and somatostatin (SOM) are markers for two of the most common, distinct, and well-characterized groups of inhibitory interneurons in the cortex. Studies in the somatosensory cortex have divided SOM interneurons into subgroups based on electrophysiology (Halabisky et al. 2006), but the fine structure of variations in electrophysiology of the fast spiking `basket' type PV interneurons have not been defined. This thesis provides a method for objective classification of interneurons based on passive and active firing properties, and verifies the effectiveness of the method by blindly classifying PV and SOM interneurons into the correct groups. Using this method, four PV interneuron subgroups are identified in layers 2/3 of the visual cortex. Independent support for this classification is provided by the fact that the subgroups also differed in excitatory synaptic input. These subgroups would each be capable of fulfilling a distinct functional role in the neural circuit.