Introduction: Photostability Testing of Drugs and Drug Formulations - Why and How? Photophysical and Photochemical Aspects of Drug Stability. Standardization of Kinetic Studies of Photodegradation Reactions. Rationalizing the Photochemistry of Drugs. Technical Requirements and Equipment for Photostability Testing. Photostability Testing: Design and Interpretation of Tests on New Drug Substances and Dosage Forms. The Questions Most Frequently Asked. Inconsistencies and Deficiencies Which Exist in the Current Official Regulations Concerning the Photolytic Degradation of Drugs. Benefits and Adverse Effects from the Combination of Drugs and Light. In Vitro Screening of the Photoreactivity of Antimalarials. A Test Case. Screening of Dyes, Drugs and Dietary Supplements for Ocular Phototoxicity. Photochemical and Photophysical Methods Used in the Study of Drug Photoreactivity. Addressing the Problem of Light Instability During Formulation Development. Photostability of Parenteral Products. Photoactivated Drugs and Drug Formulations. Formulation Approaches for Improving Solubility and its Impact on Drug Photostability. Appendices.