Genetically based host resistance is an attractive proposition from the point of view of the grower of improved stock. The grower, apart from paying for the resistant genetic stock, does not have to make further major outlays, although management practices that allow the resistance to be expressed may be required. A further advantage of host resistance is the minimal negative impact on the environment as neither antibiotics nor chemical treatments are normally needed to enhance control. On the other hand development of genetically based host resistance is often costly and may be impossible to achieve in the absence of useful levels of resistance. Furthermore, each characteristic that is added to a selective breeding program inevitably leads to slower progress in other desirable characteristics in the breeding goal. Added to this, disease resistance may be negatively associated with other desirable characteristics. These associations may be genetically linked to genes that are close to each other on the same chromosome, or there may be a metabolic, physiological or ecological cost of the resistance. Selective breeding for resistance is an attractive option for managing diseases, but it is not a panacea for control of all diseases. In order to justify the high cost of developing genetically disease resistant populations this approach is only advisable when: (a) the disease causes severe damage; (b) there are no other existing simple cost effective control measures; and (c) there is demonstrable genetic variation in resistance and this is not coupled with an excessive level of negative associations with other desirable characteristics.