Biofilm is known as a community of single or multi species of microorganisms, including bacteria. Bacteria in biofilms have increased resistance to antimicrobials that may reach 10 to 1000 times more than the minimal inhibitory concentrations required for free-living bacteria. The increased antimicrobial resistance is thought to be due to pathways other than the conventional resistance mechanisms seen in free-living (planktonic) bacteria. There is no single general mechanism that explains biofilm resistance to antimicrobials; it is rather a complex process that involves a growing list of many factors. Several studies support the increasing number of resistance mechanisms, which are categorized in six classes: extracellular matrix, altered microenvironment, stress response, quorum-sensing, presence of persisters, and bacterial outer membrane proteins; in addition to other mechanisms that do not fall under these six general categories. A full understanding of biofilm resistance mechanisms will help researchers to develop effective treatment strategies for eradicating biofilm-based infections in the future.