Since the passage of the No Child Left Behind Act of 2001, standardized test scores have revealed that the U.S. public education system has been unable to adequately address improvement of academic achievement. For 20 years, educators have been promoting professional learning communities (PLCs) as a solution to improving K-12 academic achievement. Many PLCs are in effect throughout the U.S.: however, there has been a wide disparity of effectiveness in the implementations of PLCs in schools. The research literature indicated the need for more support from the principal for teachers to increase motivation and productivity, yet there is a scarcity of information on the relationship between principal support and perceived effectiveness of PLCs. Consequently, the purpose of this quantitative study was to examine teachers' perceptions of this relationship. A self-administered, online, Likert-scale instrument was used to determine if there was a correlation between each of nine independent variables and perceived effectiveness of PLCs. The primary independent variables were principal's support of teacher empowerment and principal's support of resources, together with components of these variables. The remaining independent variable represented the combination of the primary independent variables. The population size was 700, and the sample size from a total of 21 schools consisted of 224 teachers who belonged to a PLC in participating elementary and middle schools. A Spearman rho correlation revealed a statistically significant relationship between principal support of teacher empowerment and perceived effectiveness of PLCs (rs[224] = 0.846, p less than 0.001). In addition, a Spearman rho correlation revealed a statistically significant relationship between principal support of resources and perceived effectiveness of PLCs (rs[224] = 0.830, p less than 0.001). Moreover, a Spearman rho correlation revealed a statistically significant relationship between teachers' perceptions of principal support of teacher empowerment and resources combined, and perceived effectiveness of PLCs (rs[224] = 0.856, p less than 0.001). These findings suggested that principal support of teacher empowerment and resources plays a role in the effectiveness of PLCs. A recommendation for future research is to conduct this study in high schools. [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://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]