Low probability of intercept (LPI) is the term used for there being a low probability that a radar's emissions will be usefully detected by an intercept receiver in another aircraft or on the ground. For the air battle of the future, LPI is essential. In conventional aircraft the most important need for LPI is to avoid electronic countermeasures. In low observable aircraft, LPI additionally enhances the element of surprise and denies the enemy use of radar intercept queuing of its fighters. In aircraft of both types, LPI prevents successful attacks by antiradiation missiles. In this chapter, we will review the generic types of intercept receivers and see what strategies may be used to defeat them. We'll then take up specific design features which may be incorporated in a radar to ensure a low probability of intercept. Finally, we'll very briefly assess the cost of LPI and consider possible future trends in LPI design.