Co-evolution between bacteria and phages is the most intense evolutionary process on our globe. Phage infection is initiated by the recognition of a specific receptor located on the bacterial cell by the phage’s receptor-binding protein (RBP). In response to the high capsule serotype diversity, Klebsiella pneumoniae phages have evolved RBPs comprising serotype-specific depolymerase domains that degrade the capsule as receptor. I will focus on the evolution of RBPs from two model Klebsiella phages that I have studied in depth in terms of RBP functioning and organization in the past few years: phage KP32 and phage KP34. While both phages have a similar dual RBP system, KP32 is bivalent and KP34 monovalent phage (infecting two capsule serotypes versus one). Using synthetic biology tools and phage training, I will investigate RBP evolution driven by phage fitness and serotype availability. First, natural RBP evolution on domain level will be mapped. Secondly, the influence of single residues on serotype specificity of the RBPs, shaped by vertical evolution, will be elucidated. Finally, the concerted action of both horizontal and vertical evolution to tune phage specificity and fitness will be studied. With my fundamental research I aim to deliver deep knowledge on RBP systems and their evolution, which can be translated to the construction of effective synthetic phages with a programmed host specificity, applicable in therapy against multidrug-resistant bacteria.