Studies of category-specific knowledge with brain-damaged patients and fMRI have opened new vistas on our understanding of the neural basis of semantic memory. Unfortunately, this work leaves several unanswered questions. I will present some preliminary work investigating the processes that contribute to categorization in semantic memory. There appear to be at least two semantic categorization processes: Similarity-based categorization, involving a relatively rapid, perceptually-based comparison to previously encountered exemplars (or possibly a prototype) of a category; and rule-based categorization, involving a slower, deliberate, resource-dependent evaluation of specific features to determine category membership. In one study, I describe some investigations of semantic categorization in Alzheimer's disease. While healthy seniors are able to use either rule-based categorization or similarity-based categorization to make decisions about the category membership of object descriptions, patients with Alzheimer's disease are able to use only a similarity-based categorization process. We find a similar pattern of semantic categorization restricted to similarity-based processing in patients with frontotemporal dementia, implying a role for frontal brain regions in rule-based categorization. In an fMRI study using the same materials, we find significantly greater activation of left dorsolatearal prefrontal cortex during rule-based semantic categorization compared to similarity-based semantic categorization. These findings appear to suggest that limitations in semantic categorization processes contribute to semantic memory difficulties in patients with Alzheimer's disease, that semantic memory is a multi-faceted process involving both the representation of content and the processes that organize this content, and the prefrontal cortex contributes to rule-based semantic categorization.