Background: Nursing philosophy is the foundation of nurse practitioner (NP) training. However, NP practice is based on the medical care model. Thus, the necessity of mediating between these two approaches is often problematic for new NPs who are transitioning into their new roles. Mentoring has been used successfully to facilitate role transition and role understanding for nurses, NPs, and physicians. However, mentoring has been rarely studied in NPs. Purpose: The purpose of this study was to develop a theory of mentoring for new NPs in a hospital setting. Methods: Grounded theory methodology was used. The sampling approach was initially purposive and was then shifted to theoretical to ensure the collection of meaningful data. Semistructured interviews were recorded and transcribed into Word documents for analysis. The three-phase analysis developed by Corbin and Strauss was initiated after the second interview. Results: Sixteen participants (eight mentors and eight mentees) were interviewed between February and June 2011. The core category that emerged from the datawas Bdefining self,[ and the main categories were forming the relationship, developing the relationship, and mentoring outcomes. Conclusions: A well-designed formal mentoring program may greatly improve the transition of NPs into a newrole. The theory generated by the data from these study participants provides clearly defined categories that may be operationally defined and utilized to develop evaluation tools for mentoring programs.