The article "Interneuron Cell Types: Fit to Form and Formed to Fit" by Adam Kepecs and Gordon Fishell reviews the complexity and diversity of interneurons in the forebrain, particularly in the cortex. Interneurons, despite being a minority in the brain, play a crucial role in controlling inhibition. The authors argue that while interneuron diversity has been a central focus of neurobiologists, it has defied a generalized classification system. Recent studies suggest that this diversity can be simplified by viewing interneurons as elaborations of a finite group of developmentally specified cardinal classes. The article discusses the developmental origins of interneurons, highlighting that they arise from specific embryonic subcortical progenitor zones, such as the medial and caudal ganglionic eminences (MGE and CGE). These zones give rise to complementary interneuron subtypes, with the MGE producing parvalbumin (PV)-expressing fast-spiking interneurons and somatostatin (SST)-expressing populations, and the CGE producing neurogliaform, bipolar, and VIP-expressing multipolar interneurons. The authors also explore the role of local cues in the differentiation and connectivity of interneurons, suggesting that activity-regulated gene expression during critical periods may contribute to the formation of specific subclasses. They propose a two-phase model of interneuron specification, where developmental genetic programs establish cardinal classes, and later interactions with other neurons refine these classes. The functional aspects of interneurons are discussed, focusing on their roles in cortical computations, including arithmetic operations and timing. Recent studies using optogenetics have shown that specific interneuron types, such as PV and SST-expressing neurons, can modulate visual responses and control spike timing in hippocampus. The article concludes by emphasizing the importance of understanding the developmental and functional properties of interneurons to advance our understanding of neural circuits and their computational functions. The authors suggest that a framework based on cardinal classes can help direct future research and provide a foundation for exploring the diverse roles of interneurons in the brain.The article "Interneuron Cell Types: Fit to Form and Formed to Fit" by Adam Kepecs and Gordon Fishell reviews the complexity and diversity of interneurons in the forebrain, particularly in the cortex. Interneurons, despite being a minority in the brain, play a crucial role in controlling inhibition. The authors argue that while interneuron diversity has been a central focus of neurobiologists, it has defied a generalized classification system. Recent studies suggest that this diversity can be simplified by viewing interneurons as elaborations of a finite group of developmentally specified cardinal classes. The article discusses the developmental origins of interneurons, highlighting that they arise from specific embryonic subcortical progenitor zones, such as the medial and caudal ganglionic eminences (MGE and CGE). These zones give rise to complementary interneuron subtypes, with the MGE producing parvalbumin (PV)-expressing fast-spiking interneurons and somatostatin (SST)-expressing populations, and the CGE producing neurogliaform, bipolar, and VIP-expressing multipolar interneurons. The authors also explore the role of local cues in the differentiation and connectivity of interneurons, suggesting that activity-regulated gene expression during critical periods may contribute to the formation of specific subclasses. They propose a two-phase model of interneuron specification, where developmental genetic programs establish cardinal classes, and later interactions with other neurons refine these classes. The functional aspects of interneurons are discussed, focusing on their roles in cortical computations, including arithmetic operations and timing. Recent studies using optogenetics have shown that specific interneuron types, such as PV and SST-expressing neurons, can modulate visual responses and control spike timing in hippocampus. The article concludes by emphasizing the importance of understanding the developmental and functional properties of interneurons to advance our understanding of neural circuits and their computational functions. The authors suggest that a framework based on cardinal classes can help direct future research and provide a foundation for exploring the diverse roles of interneurons in the brain.