Intra-abdominal infections (IAIs) are commonly encountered in clinical practice. The etiology of these infections, often polymicrobial in nature, can be variable and usually includes organisms derived from the gut microbiota. in community-acquired IAIs enterobacteria predominate (mostly Escherichia coli) in combination with anaerobes (mostly Bacteroides fragilis). In nosocomial IAIs, which can complicate abdominal surgery, other pathogens can also play a role, such as Pseudomonas aeruginosa, Acinetobacter baumannii, Staphylococcus aureus, Enterococcus spp. and Candida spp. Diagnostic microbiology of IAIs is complex and plays a relevant role, especially in some situations (e.g. presence of foreign bodies, potential presence of resistant or uncommon pathogens, nosocomial infections in subjects with risk factors). Antibiotic resistance issues are currently encountered in most pathogenic species causing IAIs. Resistance affects all major classes of antimicrobial agents, often involving multiple classes and resulting in complex resistance phenotypes for which only a very limited number of drugs remain active.