Oncolytic viruses (OVs) are a class of viruses that can selectively replicate in and lyse cancer cells, and subsequently spread with a tumor while not causing damage to normal cells. Besides the direct oncolytic activity, OVs are also effective in stimulating the body's immune responses which can further help recognize, control, or destroy cancer cells. Through the combination of tumor-specific cell lysis and immune stimulation, OVs act in their antitumor activities in situ. OVs encompass a broad diversity of DNA and RNA viruses and typically fall into 2 classes: (1) viruses that are naturally cancer-selective such as reovirus, and (2) viruses that are genetically manipulated including measles virus, adenovirus, and herpes simplex virus (HSV-1), and so on. To date, the effectiveness of OVs has been supported by many preclinical data. A few oncolytic viruses have been approved by the Food and Drug Administration in the United States. The OV approach to cancer treatment is becoming more interesting for scientists.