Among the circumstances that may cause a too late or erroneous diagnosis of skin cancer, a most important one is the failure to distinguish a true squamous cell carcinoma of the skin from an atypical, but benign, epidermal hyperplasia which closely mimics the carcinoma clinically and particularly histologically - so-called pseudoepitheliomatous or pseudocarcinomatous hyperplasia. Although there are many publications describing in detail the histologic features of pseudocarcinomatous hyperplasias which arise in various dermatoses or skin tumors, this book appears to be the first attempt to cover the whole subject of pseudocarcinomatous epidermal hyperplasia by considering it not only in its clinical and histopathologic aspects, but also from cytophysiologic and cytogen- etic points of view, in relation to squamous cell carcinoma. On the basis of a review of the available literature and of my own correlative clinical, histopathologic, cytophysiologic, and cytogenetic investigations in patients and experimental animals, I propose a comprehensive set of criteria of difference and similarity between pseudocarcinoma and true carcinoma of the skin. Hopefully, these criteria will be found useful by physicians in the differential diagnosis between these conditions in cases of doubt and may likewise provide a stimulus to researchers for deliberation. I also hope that this monograph, which is a translation of a revised version of the text originally published in Russian in 1980, will serve our common cause of the fight against cancer. I welcome suggestions regarding the choice and treatment of the material included in the book.