Understanding of a clinical trial is essential in developing clinical guideline and adopting evidence based practice. In designing and executing clinical trials, following ethical requirements should be considered : social value, scientific validity, fair subject selection, informed consent, favorable risk-benefit ratio, institutional review board, and respect for human subjects. According to the stage of drug development, purpose of trials, accumulated scientific data, clinical trials for drug development are classified as phase 1, 2, 3, and 4. Phases of clinical trials can be overlapped and the judgment of entering into the next phase should be considered highly strategically. In reading, evaluating and interpreting clinical trial reports, various skills and challenges exist. Patient sample composition, trial duration, selection of endpoints, responders and non-responders, placebo effect, patient recruitment, and extrapolation to the real world are the examples of those challenges. Treatment success will come from the well balanced approach of evidence based decision making and consideration of specific single case.