Carbapenem resistant Klebsiella pneumoniae is one of the most clinically relevant multidrug resistant pathogens, exacerbated by an ability to cause nosocomial infections. While treatment choices are limited, the introduction of two β-lactam/β-lactamase inhibitor combinations: meropenem/vaborbactam and ceftazidime/avibactam has revolutionised the management of cephalosporin and carbapenem resistant K. pneumoniae infections. With an increasing clinical dependence on these combination therapies, there is a need to study resistance mechanisms to allow prediction of what might be seen in the clinic and perhaps suggest ways to reverse resistance when it occurs. In particular, this project aims to better understand β-lactam/vaborbactam efficacy against laboratory grown bacteria and the mechanisms of resistance that emerge by generating mutants of K. pneumoniae clinical isolates with reduced susceptibility to β-lactams in combination with vaborbactam. We showed that vaborbactam is a potent inhibitor of class A and C β-lactamases in vivo but shows particular potency against bacteria carrying the class A, KPC carbapenemase, when used in combination with meropenem. Upon analysis of meropenem/vaborbactam resistant mutants, the outer membrane porin OmpK36 was frequently lost, as has been previously reported. We also identified a novel meropenem/vaborbactam resistance mechanism, which involves down-regulation of OmpK36 and the Maltoporin LamB2. An attempt was made to find a universal β-lactam/vaborbactam combination that would be clinically active against meropenem/vaborbactam resistant K. pneumoniae producing KPC, CTX-M or the OXA-48 like carbapenemase OXA-232. While imipenem/vaborbactam and ceftazidime/vaborbactam were active against meropenem/vaborbactam mutants of CTX-M and OXA-232 producers, an alternative combination could not be found that was active against a meropenem/vaborbactam resistant mutant derived from a KPC producer. Furthermore, ceftazidime/vaborbactam resistance was identified in CTX-M producers as being caused by over-production of the CTX-M enzyme. Imipenem/vaborbactam resistance was not identified in this background. Finally, demonstrated for the first time that combining avibactam/ceftazidime with meropenem/vaborbactam overcomes meropenem/vaborbactam resistance in all mutants generated. Therefore, we conclude that combining these β-lactam/β-lactamase inhibitor combinations in the same patient shows great potential in the treatment K. pneumoniae isolates resistant to one of these combination.