**1. Introduction**

Antibiotic resistance among clinical isolates of *Klebsiella pneumoniae*, *Pseudomonas aeruginosa*, *Acinetobacter baumannii*, *Escherichia coli*, *Enterobacter* spp and *Proteus* spp is causing worldwide concern [1-5] especially when mediated by transferable genetic elements.

The role of the Clinical Microbiology Department in this regard is crucial for the isolation of non-susceptible bacteria and the detection of the underlying mechanisms leading to their resistant phenotype. Rapid and reliable results are of the utmost importance in order to apply the appropriate treatment and to contain the spread of resistance determinants within hospital settings.

In the present chapter, laboratory procedures for the detection of antibiotic resistance mecha‐ nisms will be discussed focusing mainly on those more frequently used for Gram negative clinical isolates around the world.
