SecReT6 catalogs T6SSs by function including anti-bacterial, anti-eukaryotic, anti-adverse environment, effector translocation, adherence, and virulence. T6SSs are macromolecular apparatuses deployed as versatile ‘weapons’ to outcompete other bacteria or attack simple or higher eukaryotes. Some T6SSs are known to be involved in the internalization of bacteria into host eukaryotic cells. T6SSs in a broad range of important pathogens have been increasingly implicated in colonization, virulence and eukaryotic cell invasion. And T6SSs have been reported to confer anti-bacterial activities by efficiently targeting competitor bacterial cells through the injection of antibacterial toxins. To date there are limited examples of experimental validation of the biological significance of T6SSs, and many hypothesized phenomena and models await further exploration.