Endophthalmitis may be caused by the microorganisms derived from conjunctival sac, contaminated devices, irrigating solutions, the implanted intraocular lens, or airborne contamination. The Endophthalmitis Vitrectomy Study has shown that in 67.7% of cases with bacterial postoperative endophthalmitis, the intraocular isolates were indistinguishable from the conjunctival and lid specimens [1]. Microbial spectrum of postoperative endophthalmitis varies in different countries and is dependent on environmental, geographical, or climatic factors. Table 5.3 presents the etiology of postoperative endophthalmitis in different regions. Gram-positive bacteria, including Staphylococcus epidermidis, Staphylococcus aureus, and Streptococcus pneumonia, are the most commonly isolated organisms from endophthalmitis occurring after cataract surgery in Europe; the gram-negative bacteria represent up to 14% of cases [9, 13, 32, 33]. There are, however, significant differences in a rate of enterococcal infections in Sweden (30–31%) and other European countries (2% in the Netherlands and UK), or the USA (3%). This shift in the preponderance of enterococcal endophthalmitis in Sweden may be connected with widespread use of intracameral cefuroxime and increased proportion of cefuroxime-resistant species. In the USA, as in Europe, the most commonly identified microorganisms are coagulase-negative Staphylococci (CONS), whereas the rate of streptococcal infections is lower [34]. In countries such as Taiwan, India, and China, the reported percentage of gram-negative and fungal cases are much higher than in Europe and the USA [35–38]. It has been shown that the bacterial virulence level is the main prognostic factor predictive of the final visual result [39].