A requirement for evaluating the effectiveness of major resistance genes (R-genes) is detailed knowledge of the genetic variability of the nematode populations in an area. Obtaining insight in the genetic variation in an area is a tour de force, because qualitative data are not sufficient allele frequency data at well studied loci are required. Another important step in predicting the durability of major R-genes is to study the parasite at the molecular level. Also for nematodes it seems that R-genes are part of a molecular surveillance system recognising foreign molecules. For potato cyst nematodes it is becoming clear that numerous proteins are secreted to manipulate the host plant and it may be assumed that some of these secretory proteins are also recognised by the host. Knowledge of the molecular nature and function of these avirulence gene products may reveal clues to predict the durability of an R-gene. R-genes that recognise avirulence gene products that have a crucial function in the nematode maybe very durable. When the current molecular models for gene-for-gene interactions are correct, the durability of an R-gene may be predicted by studying the dispensability factor and functional constraints of the avirulence gene products.