Insect baculoviruses have been genetically modified to improve their speed of kill. Whilst these viruses show considerable promise for improving crop protection, any risks that might be attached to their wide-scale release need to be assessed. The potential hazards of releasing genetically modified baculoviruses are (i) negative effects on susceptible non-target species, and (ii) movement of the introduced gene. One approach to risk assessment is to estimate the relative fitness of the genetically modified virus as compared to the wild type. Initial laboratory and field experiments have shown that both productivity and transmission are significantly reduced in baculoviruses which express insect-selective toxins. This indicates that these viruses are likely to be less fit than the parent wild type. However, further information is needed on whether genetic modification alters baculovirus persistence and how baculoviruses behave in less susceptible species and species with different life history strategies.