Nonhost resistance confers robust protection against pathogenic invaders, and has many similarities to host resistance. Through the different steps of pathogen development, plants make use of diverse defence strategies to present obstacles to the invader. These include preformed barriers, innate immunity in response to general elicitors and, as a last option, resistance mediated by independent and simultaneously acting pairs of pathogen avr and plant R genes. Our understanding of the roles played by these obstacles is relatively poor in nonhost resistance compared to host resistance. There is an obvious need to investigate how these roles may depend on the evolutionary distance between the pathogen host and a certain nonhost plant.