Oncolytic therapy is a novel anticancer treatment with attenuated lytic viruses such as adenovirus (Ad). These viruses kill the host cells through their lytic replication cycle and are thus distinct from classical gene therapy viruses, which serve as gene delivery agents and do not replicate. Oncolytic Ads are genetically engineered so as to replicate only in cancer cells. Their replication cycle leads to viral multiplication, the killing of the host cells and spreading of the infection throughout the tumor. Following success in preclinical studies, their anti-tumor potential is now being evaluated in the clinic. Three oncolytic Ads (dl1520, Ad5-CD/TKrep, and CV706) have completed Phase I and II clinical trials in cancer patients where their administration via multiple routes and in combination with chemo- or radiotherapies, has demonstrated overall safety. These viruses are being re-engineered to arm them with additional therapeutic genes, bolstering their oncolytic activity with a bystander effect. For example, Ad5-CD/TKrep delivers a therapeutic prodrug-activating (suicide) gene. These data indicate that oncolytic Ads are a promising novel cancer treatment approach that can be combined with other modalities, such as gene therapy and classical chemo- and radiotherapies. Further improvements to enhance their specificity, targeting and oncolytic activity are needed however, as these first-generation viruses showed modest anti-tumor activity. To improve their efficacy in the clinic, it will be important to devise and incorporate novel monitoring techniques in the clinical trials, such as analysis of viral replication in biopsies and through the use of creative noninvasive imaging technologies.