The fundamental difference between tumor and normal tissue growth is the emergence of the microenvironment with diminished or extinguished immunogenicity. One of the main functions of oncolytic viruses is the formation of such a microenvironment, which leads to a revival of immunological processes and loss of viability of cancer cells. Oncolytic viruses are being continuously improved and should be considered as a possible adjuvant immunomodulatory cancer treatment. A key requirement for the success of this cancer therapy is the specificity of the oncolytic viruses, which replicate only in tumor cells but do not affect normal cells. In this review, optimization strategies to achieve cancer specificity with increased efficacy are discussed and the most interesting results from preclinical and clinical trials are presented.This review provides information on the current status of the development and use of oncolytic viruses as part of the bio-logical treatment of cancer.