The semantic system contributes to the process of speech production in two major ways. The basic information is contained within semantic representations, and the semantic control system manipulates that knowledge as required by task and context. This thesis explored the evidence for interactivity between semantic and phonological stages of speech production, and examined the role of semantic control within speech production. The data chapters focussed on patients with semantic aphasia or SA, who all have frontal and/or temporoparietal lesions and are thought to have a specific impairment of semantic control. In a novel development, grammatical class and cueing effects in this patient group were compared with healthy participants under tempo naming conditions, a paradigm which is thought to impair normal semantic control by imposing dual task conditions. A basic picture naming paradigm was used throughout, with the addition of different grammatical classes, correct and misleading phonemic cues, and repetition and semantic priming: all these manipulations could be expected to place differing loads on a semantic control system with either permanent or experimentally induced impairment. It was found that stimuli requiring less controlled processing such as high imageability objects, pictures with simultaneous correct cues or repetition primed pictures were named significantly more accurately than items which needed more controlled processing, such as low imageability actions, pictures with misleading phonemic cues and unprimed pictures. The cueing evidence offered support to interactive models of speech production where phonological activation is able to influence semantic selection. The impairment in tasks such as the inhibition of task-irrelevant material seen in SA patients and tempo participants, and the overlap between cortical areas cited in studies looking at both semantic and wider executive control mechanisms suggest that semantic control may be part of a more generalised executive system.