This study examined the variations between providing secondary-school teachers with collaborative professional development (PD) and traditional PD. Although the U.S. local and federal governments spend billions of dollars on teacher PDs each year, there is still a debate on the effectiveness of those trainings. Research suggests that the most effective PDs have several features: (1) teachers can collaborate, (2) ongoing PDs are provided with more content hours for learning, and (3) learning activities are presented with clear objectives. This study examined secondary teachers' knowledge and perceptions of vocabulary instruction gained via a traditional versus a collaborative PD. The researcher conducted a five-session PD emphasizing instructional vocabulary strategies. Thirty-six secondary teachers from different content areas working in a rural southeastern high school participated. The study collected pre- and post-survey data using a quantitative research methodology. Participants were assessed through two survey instruments to determine potential changes in their (1) instructional vocabulary knowledge, and (2) their perceptions of self-efficacy regarding delivering vocabulary via effective instructional strategies. This study's independent variable (IV) was a collaborative PD intervention provided to the collaborative (COL) cohort. The business-as-usual cohort (BAU) received a traditional PD. The dependent variables (DVs) comprised participants' perceptions of self-efficacy and vocabulary content knowledge. Independent and dependent t-tests data were used to analyze differences "within" each cohort from pre- to post-surveys and "between" the two cohorts from pre- to post-surveys. The data showed cohort-wide changes in knowledge and perceptions as an outcome of each PD. When analyzing the data for the differences "within" each cohort, the study determined that both cohorts showed statistically significant gains in knowledge and perceptions from pre- to post-surveys. However, when analyzing the data for the differences "between" the cohorts, the study determined that the COL cohort's growth from pre- to post-surveys was not significantly different from the BAU cohort. Thus, the collaborative interventional PD did not substantially shape the COL cohort's knowledge and perceptions. [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.]