Topical treatment modalities have multiple advantages starting with the convenient application and non-invasive treatment and ending with the reduction of the risk of the systemic side effects. Active pharmaceutical substances must reach the desired concentration at the target site in order to produce a particular therapeutic effect. In contrast to other dosage forms topical agents applied to the skin may also be susceptible to photodegradation after application. That is why the knowledge of the susceptibility of these topical drugs to UV irradiation, which may contribute to their degradation or changes in chemical structure, is very important. Active pharmaceutical substances used in dermatology may differ both in chemical structure and photostability. Furthermore, various factors-such as light intensity and wavelength, pH, temperature, concentration-can influence the photodegradation process, which is reflected in particular in kinetics of photodegradation of active pharmaceutical substances as well as both the quantitative and qualitative composition of by-products. The aim of this study was to conduct a systematic review of the photostability of dermatological drugs, as well as of other substances commonly applied topically. The photostability of glucocorticosteroids, retinoids, and antifungal drugs as well as non-steroidal anti-inflammatory drugs applied topically and selected UV-filters have been discussed. Furthermore, the impact of photoinstability on the effectiveness of pharmacotherapy and some photostabilization strategies have been also included. the active pharmaceutical ingredient (API) must reach the desired concentration at the target site so that it provides a particular therapeutic effect. Appropriate API concentration will be obtained after the application of the recommended amount of drug product (a finished dosage form) to the skin [4]. The skin is the largest organ of the human body (1.5-2.0 m 2 ) that protects against external environmental factors, and thus is exposed to, among others, UV radiation and xenobiotics. Overcoming the barrier that constitutes the skin is possible for lipophilic substances with log P in the range of 1-3 and those with a low molecular weight (MW < 500 Da) [2]. Therefore, many substances applied topically that penetrate through the stratum corneum, reach the dermis and via dermal microcirculation could get into the systemic circulation causing systemic effects [2,3,5].The stability studies of APIs and drug products are a priority in the research and development of drugs. Information about the stability of the drug is necessary to ensure its appropriate quality, effectiveness, and safety for the patients [6,7]. General rules for conducting stability tests are included in the Guidance for Industry Q1A(R2) Stability Testing of New Drug Substances and Products which was developed by International Conference on Harmonisation of Technical Requirements for Registration of Pharmaceuticals for Human Use [8]. The ultimate goal of the stability studies is to provide industry...