The purpose of the study is to help fill the present gap in our knowledge of the internal labor market, and particularly of the internal mobility patterns of low income workers. Through the analysis of data from two samples, one drawn from New York City and the other from the entire nation, the document explores the determinants of worker "attachment" and the patterns and degree of upward mobility. The document includes a summary of the literature on internal mobility and related questions, discusses the data used (derived from the Continuous Work History Sample of the Social Security Administration), presents findings on the attachment and advancement patterns of low-income workers, and analyzes mobility patterns in three New York City industries (banking, general merchandising stores, and apparel manufacturing). Major findings are summarized in terms of attachment patterns as related to sex, race, and age and advancement patterns. Findings noted as significant were the high degree of firm and industry attachment exhibited by females, particularly in the garment industry, and the success of males in raising their incomes. Implications for manpower policy, and suggested directions for further research conclude the document. Appended is a four-page bibliography. (Author/BP)