Abstract While the competencies and theory relevant to public health nursing (PHN) practice continue to be described, much less attention has been given to the knowledge derived from practice (clinical know-how) and the development of PHN expertise. A study was designed to address this gap by recruiting nurses with varied levels of experience and from various practice sites. A convenience sample of 28 public health nurses and seven administrators/supervisors were interviewed. A subsample, comprised of less-experienced public health nurses, were followed longitudinally over an 18-month period. Data included more than 130 clinical episodes and approximately 900 pages of transcripts and field notes. A series of interpretive sessions focused on identifying salient aspects of the text and comparing and contrasting what showed up as compelling, puzzling, and meaningful in public health nurses' descriptions. This interpretive analysis revealed changes in understanding of practice and captured the development of clinical know-how. In Part 1, we describe the sample, study design, and two aspects of clinical knowledge development—grappling with the unfamiliar and learning relational skills—that surfaced in nurses' descriptions of early clinical practice. In Part 2, which is to be published in the next issue of Public Health Nursing (SmithBattle, Diekemper, & Leander, 2004), we explore gradual shifts in public health nurses' understanding of practice that led to their engagement in upstream, population-focused activities. Implications of these findings for supporting the clinical learning of public health nurses and the development of expertise are described.