Although there is a growing research interest in teacher informal learning and an increasing acknowledgment of the important role it plays in teacher professional development (Jurasaite-Harbison, 2008; Jacobs & Park, 2009; Lecat et al., 2020), the effects or outcomes of informal learning are difficult to measure and there are relatively fewer studies that focus on these outcomes (Kyndt et al., 2016). The purpose of this explanatory, sequential mixed methods study (quan [right arrow] QUAL) was to investigate math teachers' perceptions of the effects of informal learning on teacher knowledge and practice. Phase one of the study involved examining how math teachers' participation in four different informal learning activities (learning through media, colleague interaction, individual reflection, and stake-holder interaction) relate to their self-reported efficacy in select TPACK domains (technological knowledge (TK); content knowledge (CK); pedagogical knowledge (PK); and technological pedagogical content knowledge (TPACK)) and differed based on teacher career stage. The quantitative analysis was based on 258 cases with complete responses out of a total of 397 respondents who took the survey. In phase two, two rounds of semi-structured interviews were conducted, and a mini survey capturing teachers' daily informal learning experiences was administered to examine math teachers' experiences and perceptions of informal learning and explain the quantitative results obtained in phase one of the study. The qualitative analysis was based on a multiple case study approach involving four purposefully selected cases from the first phase of the study, and 173 responses from two open-ended survey items on the main survey. The research questions were answered based on meta-inferences obtained from both the quantitative and qualitative phases. This study found positive correlations between math teachers' participation in informal learning and their self-reported efficacy in select TPACK domains (TK, CK, PK, TPACK). Furthermore, math teachers' media interaction, interaction with colleagues, and participation in individual reflection were all found to be significant predictors of their self-reported efficacy in at least one of the four informal learning activities this study focused on. No statistically significant differences were found in participation in informal learning activities based on teacher career stage. Findings also revealed five distinct categories of informal learning activities that mathematics teachers engage in: consulting information sources, interaction and discussion with others, learning from others without interaction, practicing and testing, and reflection in and on action. Overall, reflecting on practice and using social media were the two most frequent informal learning activities that most math teachers reported engaging in. However, the case analysis suggested that referencing web-based resources and collaboration with colleagues may be the most prominent and useful informal learning activities in teachers' everyday practice. Three categories or levels of factors that affect teachers' informal learning were also identified: personal, interpersonal, and school-level factors. An overwhelming majority (70.3% of survey quotes) of issues that affected math teachers' informal learning were school-level factors. Time and scheduling accounted for most of these (36.9% of survey quotes), making it the most influential issue affecting math teachers' informal learning. Overall, informal learning was found to contribute meaningfully to math teachers' content, pedagogical, and technological knowledge, as well as their motivation and emotional well-being. Most contributions or effects of informal learning reported pertained to teachers' pedagogical knowledge. Teachers' TPACK self-efficacies in different domains mostly determined the areas of practice where they sought out informal learning opportunities. Although teachers found informal learning very useful for solving immediate or in-the-moment problems of practice, informal learning also contributed to changes of a more enduring nature in how teachers attended to equity and approached student engagement in their classrooms. [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.]