In attempts to understand the functional circuits in cortex and thalamus, it is important to recognize that components of the predominant excitatory circuits, which use glutamate as a neurotransmitter, are not functionally homogeneous, as they are often mistakenly represented. Instead, these glutamatergic pathways can be divided into two distinct classes: Driver, which carries the main information between cells, and Modulator, which modifies how driver inputs function. Identifying which glutamatergic pathways are which is an important prerequisite to further understanding of circuit function. Examples of how this can be applied to thalamic and cortical circuitry will be discussed.