For a number of decades, sociologists have been analyzing various crimes in terms of the concept of occupation. While both professional theft and serious theft have been distinguished from petty crime by such characterization, occupational studies have focused on the professional thief. For this study, 40 serious thieves, aged 18-23, from working-class families in Toronto were observed and interviewed. Serious thieves were defined as those persons who were recognized and labelled as such by themselves and their peers, who were officially convicted by courts or labelled by police as thieves, who had gained at least $500 in profit from theft within a 2-year period or less, and who had made at least $100 or at least one-third of their total income from theft during any single month in which they were actively thieving. All those studied were male, Anglo-Saxon or French-Canadian, and had economic problems. They were studied in terms of the areas of specialization (shoplifting, commercial burglary, clouting, stealing goods in transit, etc.) their interaction with victims, and their interaction with customers or fences. The evidence supports the hypothesis that serious thieves operate according to an occupational perspective, but not in the same way as do professional thieves. Serious thieves are younger, have less highly developed skills, are nonmigratory, spend more time in jail, and make less money than professional thieves. Serious theft seems to be age-specific and of short duration. One table accompanies the text, and four footnotes follow. A bibliography of 57 references is appended.