This dissertation develops and evaluates the validity and reliability of the Skills Self-Efficacy Scale for Adolescents (SSES-A), a measure designed to assess adolescents' self-efficacy in critical future skills: problem-solving, communication, creativity, collaboration, and critical thinking. Anchored in Social Cognitive Theory and Psychometric Theory, this research progresses through an exploratory sequential mixed-methods methodology across three phases, incorporating an empirically supported seven-step instrument development process. In Phase One, a comprehensive literature review, focus group adolescent participants (n=23), and qualitative survey (n=30) participants generated a rich item pool by articulating real-life skill applications as they related to the four sources of self-efficacy, culminating in the preliminary SSES-A draft. Phase Two engaged expert reviewers (n=35) in content validity assessment using Lawshe's technique for essentiality, relevance, and clarity, which refined the SSES-A into its second draft. This phase also included cognitive interviews with the target demographic (n=40), enhancing the scale's face validity and leading to an improved third SSES-A draft. The third and concluding phase involved pilot testing and psychometric evaluation with 385 high school students from four Connecticut public schools, employing Exploratory Factor Analysis and Cronbach's alpha. The study resulted in the SSES-A demonstrating robust construct validity, achieving a theoretical, content, and statistical balance. Additionally, the research process introduces a factor structure diagram model for future research applications. The methodical approach to the instrument development process highlights the SSES-A as a valid and reliable tool for measuring adolescent self-efficacy in essential skills for adolescent success and post-secondary readiness. [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.]