The tomato cell surface receptor-like protein (RLP) Cf-4 confers resistance to Avr4-secreting strains of the fungus Cladosporium fulvum, which causes tomato leaf mould disease. Cf-4 constitutively interacts with the receptor-like kinase (RLK) SOBIR1 and interacts with the RLK BAK1 upon recognition of Avr4 by Cf-4. Formation of the Cf-4/SOBIR1/BAK1 complex is proposed to trigger phosphorylation of the intracellular kinase domains of SOBIR1 and BAK1, which subsequently activate downstream signalling resulting in plant immunity. However, components involved in regulating the Avr4-induced formation and subsequent activation of the Cf-4/SOBIR1/BAK1 complex remain unknown. In addition, downstream components required for Cf-4 signalling are largely unknown. The work described in this thesis is aimed at gaining more insight into the molecular mechanisms of Cf-4 signalling. Furthermore, the application of the gained knowledge to genetically engineer plant immunity is pursued. Additionally, as SOBIR1, which is functionally required for all RLPs mediating immunity, also interacts with the RLP CLAVATA2 (CLV2) involved in development, the significance of the CLV2/SOBIR1 complex is investigated.