Pathogen-derived resistance (PDR) was the basic concept whereby the first virus resistant transgenic plants (VRTPs) were obtained more than a decade ago using the coat protein gene of Tobacco mosaic virus (TMV). Since then, many viral sequences have been engineered into the genome of a no. of crops making them tolerant or resistant to specific viruses for which no corresponding natural resistance genes had been found. Although this represents an indisputable advantage, it is also reasonable that occurrence of potential reassortment (for viruses with segmented genome) and recombination, between endogenous sequences and those of an incoming virus should be taken into consideration. This would affect relative reproductive ability (fitness) of the viral population in a defined environment through the potential emergence of strains with novel epidemiol. traits and more damaging properties. The consensus is that the correct baseline for risk assessment is the current situation in the absence of VRTPs. An account is given of the most recent studies on population dynamics of some economically important plant viruses with the aim to discuss whether deployment of VRTPs would present risks different from those already present in the agricultural environment.