Objectives. This study examined factors associated with emergency department use among homeless and marginally housed persons. Methods. Interviews were conducted with 2578 homeless and marginally housed persons, and factors associated with different patterns of emergency department use were assessed in multivariate models. Results. Findings showed that 40.4% of respondents had 1 or more emergency department encounters in the previous year; 7.9% exhibited high rates of use (more than 3 visits) and accounted for 54.5% of all visits. Factors associated with high use rates included less stable housing, victimization, arrests, physical and mental illness, and substance abuse. Predisposing and need factors appeared to drive emergency department use. Conclusions. Efforts to reduce emergency department use among the homeless should be targeted toward addressing underlying risk factors among those exhibiting high rates of use. (Authors)