In the majority of natural situations in which bacteria are found, they are associated with and attached to surfaces. In the presence of moisture and nutrients, they grow to form extensive bacterial films which are often enveloped within copius exopolymeric matrices. Biofilms are ubiquitous to many different situations in industry, the environment and medicine. Their presence can be either beneficial or more commonly detrimental to such systems. In this respect, biofilm populations possess physiological properties distinct from those of unattached, planktonic bacteria. Moreover, it is generally accepted that bacteria growing within a biofilm are more resistant to antimicrobial agents than their planktonic counterparts. However, although the consequences of attachment to antimicrobial resistance have been known for many years, the mechanistic bases for such effects have still to be fully elucidated. In this article the nature of different resistance mechanisms, including those of the exopolymeric matrix, environmental modulation, attachment-specific physiologies and quorum sensing are reviewed.