Professional Learning Communities (PLCs) have emerged in the last 20 years as a popular professional development initiative. However, despite a wealth of PLC literature available, researchers have found that forming teacher communities does not automatically result in improved teaching practices that support student learning. This qualitative study was used to explore the ways in which humanities teachers perceive their work in existing PLCs. Specifically, it examined the ways teachers experience their work together related to subject knowledge, pedagogical knowledge, and the influence the work within the PLC has on changing instructional practices. Using an embedded unit case study design with each PLC representing a unit, data were gathered from a sample of 19 English, social studies, and world languages teachers who belonged to five PLCs that met regularly during the school day in a Connecticut high school. The researcher completed an interview with each teacher, the high school principal, and the high school assistant principals (n = 23) and each of the five PLCs were observed three times for the duration of a full meeting block (81 minutes). Interview transcripts, participant observations, and field notes were coded to allow themes to emerge within and across cases, triangulation of the data was essential for developing themes. The researcher concluded that although the feelings about peer collaboration in PLCs were positive, teachers believed their work was limited because they needed training in several areas. This was communicated through interviews and supported by observations that identified several areas of insufficiency. To increase the likelihood that the PLCs make an impact on improving teaching practices and student achievement, it is recommended that the district develop a shared vision about PLC work, increase the participation from school leaders around the PLC work, and provide teacher training on the use of data and on the dynamics of group discourse. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]