This chapter focuses on the physiological mechanisms that arrest pathogen development or simply prevent infection. Cell-autonomous plant defences comprise two layers, namely the constitutive defences, which are physical barriers and antimicrobial factors produced during the course of normal plant growth and development, and the induced defences activated when the plant perceives that it is under attack. The latter comprise plant immunity and include mechanisms for strengthening the physical barriers at the anatomical and cellular levels, and antimicrobial factors, including both metabolites and proteins, mechanisms for disarming specific pathogenicity factors (such as toxins and effectors) as well as programmed cell death, commonly termed the hypersensitive response. There is considerable overlap between the constitutive and induced responses.