This chapter addresses the Gram stain and cellular morphology of bacteria, which are key features for preliminary identification of the Gram-negative genera. Reference laboratories may utilize mass spectrometry or polymerase chain reaction assays to assist in the identification of aerobic Gram-negative bacteria.Additionally, 16S rRNA gene sequence analysis can be used to identify unusual members of this group that give problematic results. Laboratory protocols for the identification of unknown aerobic Gram-negative bacteria should also include procedures for the safe handling of potential pathogens. Identification schemes for aerobic Gram-negative organisms that grow on TSA with 5% sheep blood agar begin with performing a Gram stain, observing colony morphology, and assessing the organism's ability to ferment glucose and grow on MacConkey agar. Molecular analysis is becoming an increasingly important tool for the clinical bacteriologist, especially in the identification of an aerobic Gram-negative bacteria.