The shift towards Competency-Based Medical Education (CBME) represents one of the most significant reforms in health professions education in recent decades. CBME emphasizes outcomes, learner progression, and demonstrable competence rather than time-based training. While this shift offers substantial educational benefits, it also places unprecedented demands on assessment systems. Designing defensible assessment systems is therefore central to the credibility, effectiveness, and sustainability of CBME. A defensible assessment system is one that is educationally sound, transparent, fair, and capable of withstanding academic, regulatory, and legal scrutiny. In CBME, where high-stakes decisions about progression and certification are based on multiple data points over time, assessment systems must be carefully designed and rigorously implemented. One of the foundational principles of defensible assessment in CBME is alignment. Assessments must be explicitly aligned with defined competencies, learning outcomes, and expected professional roles. This begins with clear articulation of competencies and entrustable professional activities, followed by systematic blueprinting of assessment tools to ensure comprehensive coverage. Without such alignment, assessment decisions risk being arbitrary or inconsistent. CBME requires a move away from reliance on single, summative examinations towards a programmatic approach to assessment. Programmatic assessment involves the collection of multiple low-stakes assessments across different contexts, assessors, and time points. These data are then synthesized to make informed decisions about learner progression. Designing such systems demands careful planning to balance feasibility with educational richness. Workplace-based assessment plays a central role in CBME. Tools such as mini-clinical evaluation exercises, direct observation of procedural skills, case-based discussions, and multisource feedback provide valuable insights into real-world performance. However, for these tools to contribute to a defensible system, faculty must be trained in their appropriate use, including observation skills, judgement calibration, and constructive feedback. Faculty development is, therefore, a critical enabler of defensible assessment systems. Assessors must understand the purpose of assessment in CBME, the distinction between formative and summative functions, and the importance of narrative feedback. Inconsistent assessor practices are a major threat to defensibility; structured training and ongoing calibration are essential to mitigate this risk. Documentation and data management are equally important. Defensible assessment systems rely on systematic collection, storage, and analysis of assessment data. Portfolios often serve as a central repository, enabling longitudinal tracking of learner progress. Transparent documentation not only supports educational decision-making but also provides evidence in the event of appeals or external review. Another key element is the use of committees or competence review boards to make high-stakes decisions. Rather than relying on individual examiners, defensible systems distribute responsibility across trained groups who review aggregated data. This collective judgement enhances fairness, reduces bias, and strengthens the credibility of decisions. Importantly, defensibility should not be equated with rigidity. A well-designed CBME assessment system remains flexible, responsive to learner needs, and open to continuous improvement. Regular review of assessment data, stakeholder feedback, and outcomes allows institutions to refine tools and processes over time. In conclusion, designing defensible assessment systems in CBME is a complex but essential undertaking. It requires alignment, programmatic design, faculty development, robust documentation, and collective decision-making. When done well, such systems not only withstand scrutiny but also promote meaningful learning, professional growth, and public trust in health professions education.