Objective: We examine how mothers' characteristics are associated with forming a doubled-up household as a host (allowing adult extended family members/nonrelatives to join their household) and guest (moving into a home owned/rented by extended family/nonrelatives). Background: Doubled-up households are increasingly common and shape families' lives in meaningful ways. Although doubling up is often considered a response to economic need, few studies directly examine the range of characteristics that may predict entry into doubled-up households. Method: Using data from the Survey of Income and Program Participation, we examine how demographic, economic and housing, and family characteristics are associated with mothers' risk of becoming doubled-up as hosts and guests. Results: Multiple factors have independent links to doubling up, including race/ethnicity, housing assistance receipt, and socio-economic status. Family factors are especially important; in particular, experiencing a new birth and having a young child are associated with doubling up as a guest, and romantic relationship changes are associated with doubling up as either a host or guest. Additionally, many predictors of doubling up as a host and guest differ. Finally, we find that almost one-third of adult mothers who are doubled-up as guests never transitioned into a doubled-up household; rather, they remained in their childhood home. Conclusion: A full accounting of when and why families double up requires researchers to attend to mothers' needs-and changes in these needs-across multiple dimensions and to distinguish between hosts and guests.