Plant defence against disease-causing organisms usually follows three steps: pathogen recognition, signal transduction and the resistance response. In tomatoes (Solanum lycopersicum), a myriad of R-genes provide resistance against viruses, bacteria, fungi and insects that counterattack with their corresponding avirulence or avr genes. R-genes have been characterised over the years and classified into four classes: TNA, CNA, RAP/RAK and miscellaneous. This book reviews these different defense genes and the recent advances associated with them.