Actinic keratosis (AK) is a very frequently diagnosed disorder in dermatological practice that mostly arises on the parts of the body that are exposed to the sun. This cutaneous lesion has been historically considered pre-cancerosis, but currently it is recommended to classify it as squamous cell carcinoma (SCC) in situ. The course of the disease is usually heterogeneous. Individual lesions may -- regress spontaneously, -- recur, -- persist without changes for a long time, or -- can progress to invasive cancer. It is known that AK and SCC of the skin represent a continuous evolutionary process that begins with ultraviolet radiation-induced damage of keratinocyte DNA, continues with clonal expansion of genetically altered cells, and ends with invasive carcinoma. The process of the malignant transformation of AK and cutaneous SCC development is similar to a mechanism of uterine cervix SCC development called cervical intraepithelial neoplasia. Therefore, an analogous classification scheme for the neoplastic changes of epidermis has been proposed named keratinocyte intraepidermal neoplasia. This 3-tiered grading system better reflects the process of gradual malignant transformation of the epidermis, but controversy still persists about its practical application. Although the malignant potential of AK is relatively low, we do not know any reliable clinico-pathological factors to predict disease outcome. That is why there is still a need for new studies focusing on the pathogenesis and biological behaviour of AK. In clinical practice, this is illustrated by the dilemma of whether all lesions unconditionally require treatment, and which is the most appropriate therapy for individual lesions. [ABSTRACT FROM AUTHOR]