Professional development (PD) is crucial for teachers to learn about new practices, technology, and improve self-efficacy to apply these skills. This sequential explanatory mixed-method study served to explore teachers' experiences with PD and using virtual labs in the classroom. First, in the quantitative phase of the study, a survey was administered to science teachers using virtual labs in private and public secondary schools across the country (n = 156). Next, in the qualitative phase of the study, the researcher conducted semistructured interviews with participants who took the survey and consented to participate in the next phase (n = 13). The researcher analyzed the survey data using correlational tests and statistical analysis to calculate the relationship between the number and type of PD and the participants' self-efficacy rating. Then, qualitative interview data were analyzed using the coding process of open coding, focused coding for emergent themes, and theoretical coding to relate the codes to the theoretical framework. Quantitative results indicate that participants who attended informal PD and three or more training sessions had significantly higher self-efficacy ratings than did participants who attended only formal PD and zero training. The qualitative findings highlighted the various experiences with PD in virtual labs, resulting in four emergent themes of peer collaboration, instructional practice, mastery of virtual labs, and personalized PD which were connected to the four aspects of self-efficacy theory. Altogether, the analyzed data indicated that informal PD significantly impacts self-efficacy, considering that it is personalized to the participant's needs and allows for peer collaboration to master the technology platform and instructional practices. [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.]