This chapter details the development of the Coaching Effectiveness Scale (CES) to provide a validated measure of coaching outcomes. Using Classical Test Theory and evidence-based principles, the chapter reports the process of defining constructs, generating items, and refining the scale. A systematic review identified seven outcome domains: career satisfaction, psychological factors, goal attainment, personal effectiveness, satisfaction with coaching, self-efficacy, and leadership/management. An initial 39-item pool was evaluated through expert review and focus groups with 33 participants, leading to a refined 24-item scale. The CES is positioned as a theoretically grounded, practically relevant tool for assessing coachee-perceived outcomes. While further validation is needed, it offers organisations a structured approach to measuring coaching effectiveness.