Indiana University-Purdue University Indianapolis (IUPUI) / Oral endocrine therapy (OET) is standard therapy for millions of estrogen receptor-positive breast cancer survivors (ER+BCS). OET reduces recurrence, mortality, and metastasis. ER+BCS often do not take their OET as recommended due to adverse side effects. The purpose of this dissertation was to develop an explanatory framework of decision making by women with ER+ breast cancer who report experiencing side effects from OET. This project was comprised of two components. The first component was a systematic review with three main findings: (1) side effects negatively impact OET non-adherence, (2) there is an absence of decisional supports provided to or available for ER+BCS who are experiencing OET side effects,, and (3) ER+BCS likely have unmet decisional needs related to OET. The second component was a grounded theory study that included 31 ER+BCS reporting OET side effects. During a single semi-structured interview, participants described the experience of OET over time. This study produced two qualitatively derived projects. First, a theoretical framework was developed that depicted four stages through which the experience of OET decision making unfolded. The stages were (1) being told what I need to do to live, (2) doing what I need to do to live, (3) enduring what I need to do to live, and (4) deciding how I want to live. Second, a typology was developed that depicted six sources of external decisional supports (healthcare providers, husbands, other breast cancer survivors, friends and family, the internet and other media sources, and God) that met four types of decisional needs (information about OET and its side effects, in-depth discussions about side effects, help in managing side effects, and emotional support). Findings can be used to develop interventions, such as decision aids, to promote quality decision making in women experiencing OET side effects.