This series of three articles is structured to provide anatomically accurate functional schematics of the motor and sensory innervation of the shoulder and upper limb. This first paper provides radiographically oriented sagittal sections through the brachial plexus to assist in directly identifying a plexal lesion. A coronal schematic of the brachial plexus and summary table allows prediction of unique patterns of denervation from 19 lesion sites. Correlation between the lesion and the denervation pattern ensures the lesion is, in fact, clinically significant. The next two articles will present a color-coded atlas that allows the radiologist to quickly assess patterns of denervated muscles and thereby indirectly localize the lesion site. Thus, the three articles can be used together to predict the clinical picture for a given nerve lesion or extrapolate lesion location when a constellation of denervated muscles are seen on an upper limb magnetic resonance imaging or electromyographic study.