The neocortex is a highly organized structure that processes diverse types of information, including sensory input, motor control, and higher cognitive functions. Despite differences in species and cortical areas, the organization of neocortical circuits is remarkably similar, suggesting a common "canonical microcircuit" that is conserved across areas and species. Neocortical neurons are classified into major classes, such as excitatory (EC) and inhibitory (interneurons), with subclasses that show conserved connectivity patterns. These classes are organized in a "serially homologous" manner, meaning that similar cell types and connections are found in different regions, adapted to the specific functions of each area. Excitatory neurons are divided into three major classes: intratelencephalic (IT), pyramidal tract (PT), and corticothalamic (CT). Each class has distinct laminar distributions and connectivity patterns. IT neurons are found in layers 2–6 and project within the telencephalon, while PT neurons project to subcerebral destinations and CT neurons primarily to the thalamus. These classes form recurrent connections with local neurons of the same class, and their connectivity is asymmetric, with a sequential organization within the local circuit. Thalamocortical (TC) projections to primary sensory cortex are classified into core and matrix types, with core-type projections to primary sensory cortices and matrix-type projections to higher-order areas. L4 neurons process extrinsic input in an area-specific manner, receiving thalamic input and projecting to L2/3 and L5A/B. L4 neurons are specialized for sensory processing, with distinct circuit properties in different modalities. IT neurons in other layers integrate signals from L4 with multiple TC and cortical inputs, forming the second stage of the local excitatory circuit. These neurons are diverse, with different subclasses showing variations in connectivity and physiology. PT neurons integrate cortical and TC inputs and project to subcerebral structures, playing a key role in broadcasting information to distant areas. CT neurons are less understood, but they receive inputs from various sources and may modulate thalamocortical activity. Inter-areal connectivity is complex, with subnetworks showing elevated interconnection and hub regions linking them. The organization of inter-areal projections may be explained by homologous IT subclasses, with differences in intrinsic physiology and connectivity patterns. Homologous inhibitory circuits mediate diverse effects on cortical processing, with interneurons such as Pvalb, Sst, and Htr3a playing key roles in modulating activity. The developmental basis of serially homologous circuits is rooted in homologous genetic programs, with transcription factors specifying cell classes and their connectivity. Differences between cortical regions arise from graded expression of transcription factors and developmental processes that shape the neocortex. L4 shows deep homology across species, with similar architectural features in different animals. The neocortex is homologousThe neocortex is a highly organized structure that processes diverse types of information, including sensory input, motor control, and higher cognitive functions. Despite differences in species and cortical areas, the organization of neocortical circuits is remarkably similar, suggesting a common "canonical microcircuit" that is conserved across areas and species. Neocortical neurons are classified into major classes, such as excitatory (EC) and inhibitory (interneurons), with subclasses that show conserved connectivity patterns. These classes are organized in a "serially homologous" manner, meaning that similar cell types and connections are found in different regions, adapted to the specific functions of each area. Excitatory neurons are divided into three major classes: intratelencephalic (IT), pyramidal tract (PT), and corticothalamic (CT). Each class has distinct laminar distributions and connectivity patterns. IT neurons are found in layers 2–6 and project within the telencephalon, while PT neurons project to subcerebral destinations and CT neurons primarily to the thalamus. These classes form recurrent connections with local neurons of the same class, and their connectivity is asymmetric, with a sequential organization within the local circuit. Thalamocortical (TC) projections to primary sensory cortex are classified into core and matrix types, with core-type projections to primary sensory cortices and matrix-type projections to higher-order areas. L4 neurons process extrinsic input in an area-specific manner, receiving thalamic input and projecting to L2/3 and L5A/B. L4 neurons are specialized for sensory processing, with distinct circuit properties in different modalities. IT neurons in other layers integrate signals from L4 with multiple TC and cortical inputs, forming the second stage of the local excitatory circuit. These neurons are diverse, with different subclasses showing variations in connectivity and physiology. PT neurons integrate cortical and TC inputs and project to subcerebral structures, playing a key role in broadcasting information to distant areas. CT neurons are less understood, but they receive inputs from various sources and may modulate thalamocortical activity. Inter-areal connectivity is complex, with subnetworks showing elevated interconnection and hub regions linking them. The organization of inter-areal projections may be explained by homologous IT subclasses, with differences in intrinsic physiology and connectivity patterns. Homologous inhibitory circuits mediate diverse effects on cortical processing, with interneurons such as Pvalb, Sst, and Htr3a playing key roles in modulating activity. The developmental basis of serially homologous circuits is rooted in homologous genetic programs, with transcription factors specifying cell classes and their connectivity. Differences between cortical regions arise from graded expression of transcription factors and developmental processes that shape the neocortex. L4 shows deep homology across species, with similar architectural features in different animals. The neocortex is homologous