Susceptibility or Immunity is the ability of the receptor or victim equipment to operate correctly in the presence of electromagnetic disturbances. Susceptibility and immunity are opposites - an equipment which has high susceptibility has low immunity, and vice versa.

Testing a device to measure its susceptibility to a disturbance requires a limit based on how a product is expected to behave during and after an event. These events, or phenomena, are either continuous or transient disturbances. A product can have a degradation of its expected performance during a transient disturbance but in no case should it become unsafe. Definitions of the performance levels are stated in the product family standards.

Immunity tests are required for CE marking to the EMC Directive and other European Directives. South Korea also has requiremetns similar to the European Union for immunity for KCC marking.