Breastfeeding is an important and relatively affordable health promotion and disease prevention activity with positive health outcomes for both the women who breastfeed and the infants who receive human milk as their first source of nutrition. As these benefits are dose dependent, the duration of breastfeeding and exclusivity of breastfeeding are important in the development of these protective effects. The purpose of this study is to understand the process younger mothers, ages 18 to 24 years, undergo to be able to sustain breastfeeding for longer than most women their age breastfeed. Ten women, ages 21 to 24 years, participated in individual, in-depth interviews in this grounded theory study. Following the iterative process of data collection and analysis, focus of the interviews changed as the data analysis guided the data collection process. Data underwent open, focused, axial, and theoretical/selective coding. The analysis process resulted in the development of a middle-range, descriptive theory grounded in the data. Prolonged Breastfeeding Through Fierce Determination succinctly explains the central phenomenon, fierce determination, and the relationship it shares with the other concepts identified as important contributing factors as the women moved through their breastfeeding journeys. Facing and Deciding to Overcome Obstacles, influenced most heavily by fierce determination, exerted its influence upon the outcome of the theory, "Trucking Along", an in vivo code provided by Participant 10, which captured her ongoing breastfeeding even in the face of obstacles. Normalized Infant Feeding Method contributed to the participants' fierce determination. Availability of Support positively impacted the participants' ability to face and overcome their breastfeeding obstacles. Personal Agency had a reciprocal relationship with fierce determination and, like availability of support, also positively impacted their ability to face and overcome obstacles. The concepts within the theory were found to be congruent with a focused review of the literature around long-term goal achievement, healthful behavior adaptation, and breastfeeding. This study contributed to the breastfeeding literature, providing insight into how young mothers engage in prolonged breastfeeding.