Adenoviruses historically have been one of the main vectors used in human gene therapy. To date, the majority of brain tumor trials of these vectors have used replication-defective viruses. The relative lack of success obtained with replication-defective vectors has prompted a search for new and improved therapies. In this context, oncolytic (conditionally replicative) adenoviruses, which selectively bind and replicate only in tumor cells, have gained increasing importance. These adenoviruses, once they are rendered conditionally replicative by transductional and transcriptional modifications, offer significant promise for patients with malignant glioma. In this review, the authors discuss the genetic approaches to adenoviral modification and their applications in the field of neurooncology.