This review reviews the ammonium/methylammonium transport (Amt) proteins of Escherichia coli and Salmonella enterica serovar Typhimurium. The Amt proteins and their homologs, the methylammonium/ammonium permease proteins of Saccharomyces cerevisiae , constitute a distinct class of membrane-associated ammonia transporters. Members of the Amt family are found in archaea, bacteria, fungi, plants, and invertebrate animals. In E. coli and serovar Typhimurium, the Amt proteins are essential to maintain maximal growth at low concentrations of ammonia, the preferred nitrogen source. Soupene and coworkers showed that a mutant of E. coli with only the low-affinity glutamate dehydrogenase pathway for assimilation of ammonia, which therefore grows slowly at low ammonia concentrations, is not relieved of its growth defect by overexpression of AmtB. A recent study on an Amt protein from tomato concluded that it was a specific transporter for NH4 + . A trimeric stoichiometry for AmtB is supported by the observation of a direct interaction between AmtB and the trimeric signal-transduction protein GlnK. In E. coli , GlnK has been observed to associate with the membrane in an AmtB-dependent fashion. Both GlnK and GlnB are sensors of nitrogen status. Their interaction with AmtB suggests a role for AmtB in nitrogen regulation. In summary, AmtB is a membrane-associated ammonia transporter that is important for growth at external concentrations of the uncharged species (NH 3 ) below about 50 nM. The preponderance of evidence suggests that AmtB specifically transports the charged species (NH 4 + ) and that this transport is passive and, hence, bidirectional.