The human brain contains a remarkable sensory system that allows us to effortlessly process speech. The processing of speech begins at our ears, where speech sounds are converted into electrical signals that propagate up to our brain. How these signals are transformed from acoustic information into meaningful speech in the human cortex is still unknown. In this dissertation, high-density human direct cortical recordings were used to systematically detail the human speech processing system and address central linguistic theories of speech perception. In Chapter 2, we examine how the superior temporal gyrus, a brain area critically implicated in speech perception, encodes phonetic features of speech. We found single site response selectivity to distinct phonetic features and distributed population encoding mediated by acoustic properties, including pitch and voice-onset-time. In Chapter 3, we examine the representation of speech sounds in human motor cortex, a region controversially hypothesized to process articulatory gestures during perception. We found evidence that motor cortex does not represent articulatory representations of perceived actions in speech, but rather, auditory vocal information. These results are consistent with linguistic feature hierarchies organized around acoustic, rather than articulatory, features. Finally, built upon the principles of human speech electrophysiology, we developed a clinical cortical mapping tool to aid in the preservation of eloquent cortex, the details of which are described in Chapter 4. All together, this work lays a foundation for understanding the human speech processing system and developing clinical applications to aid the lives of thousands with neurological disorders, including epilepsy and brain tumors.