Plant-pathogen interactions involve processes of pathogen offence, host defence and pathogen counter-attack that are commonly played out using molecules secreted by hosts and pathogens. Secreted pathogen molecules involved in these events, referred to as 'effectors', function either in the plant extracellular space (apoplast) or inside of plant cells after translocation from the pathogen. These molecules have evolved as virulence factors that can be detected by polymorphic host resistance proteins. Advances are being made in the identification and in understanding the evolution of effectors and of host uptake signals used by eukaryotic effectors to enter host cells.