Poultry can be exposed to different kinds of immunosuppressive agents that impair health and welfare by destroying innate and acquired immunity leading to diminished genetic potential of poultry for efficient production. Immunosuppression is a condition characterised by humoral and cellular immune dysfunction that leads to increased susceptibility to secondary infections and vaccine failure. Immune dysfunction at the humoral level is largely due to change in soluble factors mediated by complement or chemokines for innate immunity or due to alterations in antibodies or cytokines for adaptive immunity. In contrast, immune dysfunctions at cellular levels include alterations in neutrophils, monocyte/macrophage, and natural killer cells for innate immunity or changes in B or T lymphocytes for adaptive immunity. In poultry, stress-induced immunosuppression is manifested by failure in vaccination, and increased morbidity and mortality of flocks. Immunosuppressive agents can have cytolytic effects on lymphocyte populations leading to atrophied and depleted lymphoid organs. Immunosuppression can be due to infectious agents or non-infectious agents or due to a combination of them. At present, several modern cellular and molecular approaches are being used to determine the status of the immune system during stress and disease. Comprehensive methodologies for the evaluation of immunosuppression by combined non-infectious and infectious aetiologies have not found general application. Currently, investigations are being developed in order to detect genetic expression of immunologic mediators and receptors by microarray technology. It is likely that this new technique will initiate the development of new strategies for the control and prevention of immunosuppression in poultry. A long term immunosuppression preventive approach involves genetic selection for resistance to immunosuppressive diseases. In general, intervention approaches for immunosuppressive diseases largely rely on minimising stress, reducing exposure to infectious agents through biosecurity, and increasing immune responses by vaccination against immunosuppressive agents.