||Purpose: The purpose of this study was to explore the experience of parenting for single, unpartnered mothers of infants in the neonatal intensive care unit (NICU). The specific aims were to identify mothers' perceived stressors, methods of coping, and sources of social support. Background: The admission of a preterm or ill infant to a NICU precipitates a crisis situation for parents. During the infant's hospitalization, mothers tend to rely upon their husband or male partner for social support. However, in 2008, more than 40% of all births in the United States were to single mothers. Although many single mothers have the support of a male partner, an estimated 20% are both single and unpartnered. Because single mothers have been underrepresented in research related to parenting in the NICU, little is known about their experiences. Methods: Grounded theory methods were used. The participants were 12 single, unpartnered mothers of infants hospitalized in a Level III, 36-bed NICU. Semi-structured interviews were conducted with participants during the infant's hospitalization. Interviews were transcribed verbatim and data analysis was performed using the constant comparative method. Findings: The core variable, Forming Transitional Relationships, illustrated the varied and changing nature of the relationships that the participants experienced during their infant's hospitalization. The five theoretical categories supporting this core variable were Navigating the NICU, Developing Supportive Networks, Focusing on the Infant, Negotiating with the Infant's Father, and Redefining Self. The interrelationships among these categories form the substantive grounded theory that describes the experiences of these mothers. Conclusions: The findings of this study suggest that single, unpartnered mothers face some unique stressors during their infant's hospitalization. The transitional relationships that are formed during an infant's NICU hospitalization have the potential to be either helpful or stressful for single, unpartnered mothers. The findings of this study can inform the practice of health care professionals who work with NICU families, improve the quality of care for single, unpartnered mothers, and aid in the implementation of family-centered care.