Introduction: This study explored the development of teachers' social and emotional learning (SEL) skills by using "Teacher Effectiveness Training (TET)" (Gordon Training International) as an intervention with two groups of teachers. Further Gordon's model was approached from the perspective of modern educational psychology. The effects of TET intervention on teachers were examined by utilizing Kirkpatrick and Kirkpatrick's model, since we considered it important to look at various aspects of the outcomes of the intervention, including the participants' reactions, knowledge, knowledge application (skills) and overall well-being. Method: The teachers participating in TET were class teachers ("n" = 20) from one elementary school and subject-matter teachers ("n" = 23) from one secondary school in Finland. The comparison group comprised subject-matter teachers ("n" = 26) who did not participate in TET. The statistical differences of the post-test scores were examined with dependent sample one-way ANOVA. Results: In the comparison group, no differences between the pre- and post-test measurements were found. Among participants, "reactions" towards TET were positive. Further, there were significant results in two other aspects: both "knowledge" and "knowledge application (skills)" improved. The "overall well-being" of the teachers, measured at the end of the intervention, showed minor changes. Conclusion: TET appeared to achieve its goals, since teachers learned to apply SEL skills during the intervention.