Inputs to the thalamus display perplexing heterogeneity in source, transmitter, and the complexity of axon terminals. Almost the entire neuraxis provides excitatory and/or inhibitory terminals to the thalamus. The structure of both glutamatergic and GABAergic inputs varies from simple unisynaptic to highly complex multisynaptic terminals. Variable bouton structures support neurotransmission with different kinetics. In contrast to earlier accounts that proposed the dominance of a single type of input on thalamocortical activity (“relay cell”), in the majority of the thalamus, integration of inputs with different origins, transmitters, and complexities is the rule. Because most thalamic inputs are confined to only a portion of the structure, the emerging picture is that inputs can be integrated in many distinct ways in different thalamic territories. As a consequence, unlike in modular networks, where, however complex the input space is, it is homogeneous across the structure (e.g., the striatum, cerebellum, or cortex), no canonical thalamic module can be defined. The reason for this unique complexity is presently unclear, but the lack of canonical input organization in the thalamus certainly limits the opportunity of generalizing thalamic transfer function between territories. Deciphering the role of the thalamus requires an understanding of the diversity in thalamic input integration in each region.