Legionella spp. in groundwater sources have been reported to be the cause of waterborne infections, specifically Legionnaire's Disease and Pontiac Fever. Legionellae are known to be ubiquitous in surface waters, but much less is known about their occurrence in groundwater, which is the source of potable water for millions in North America. Federal drinking water regulations may not eliminate the threat of Legionella exposure from groundwater-derived drinking water. The primary reason is because current regulations focus on fecal contamination. Presently, there is no evidence that demonstrates an association between fecal contamination and Legionella. The study showed that (1) several species of Legionella are present in warm- and cold-system groundwater not known to be under the direct influence of surface water, (2) the water quality and ecological parameters studied showed no correlation with the presence or absence of Legionella, (3) limited Legionella-positive samples tested were also positive for several types of protozoa. While the sources of these bacteria in groundwater are not known, wide-spread Legionella colonization of the distribution systems studied suggests that treated waters from such groundwater sources may present a health risk, particularly to those debilitated or immunocompromized. This was a two-phased study. A literature review of groundwater temperatures in the United States was conducted and two warm-water sites were selected to demonstrate that Legionellae could be isolated from environmental samples. Legionellae were detected by combining a quantitative culture method (considered the gold standard) with a qualitative semi-nested PCR assay. The use of immuno-magnetic separation (IMS) was also evaluated to selectively capture Legionellae from the samples. Once the detection method was successfully demonstrated, several other warm- and cold-water sites throughout North America were surveyed for the presence of Legionellae. When combining results from the two study phases, 56 per cent of water samples (37/66) and 29 per cent of biofilm samples (20/69) were found positive by cultivation. PCR was able to detect Legionellae in 32 of the 105 (30 per cent) samples tested in Phase II; PCR inhibitors proved to be a problem, especially in biofilm samples. Based on the wells sampled, rather than individual samples, 11 of 12 (92 per cent) wells were positive for Legionellae in Phase I and 27 of 34 (79 per cent) sampled were positive for Legionellae in Phase II. Utilities with only negative samples were 0/2 in Phase I and 2/16 in Phase II. Each of these two had submitted only single samples from one well. Thus it is possible that repeated sampling or inclusion of other wells would have changed the picture. Although most of the wells were surveyed only once in this study, two wells from one utility in Phase II were re-sampled on a monthly basis for a year. In these samples it became clear that there was great temporal variability in the numbers and types of Legionellae detected. This is the first comprehensive study on the presence of Legionellae in groundwaters and its findings suggest that untreated well waters must be considered as potential sources of these opportunistic pathogens. Based on the level of contamination observed and the general assumption that all Legionella species should be considered pathogenic, we recommend that Legionella be considered for regulation in any future rules on groundwater quality. Research is also needed on the impact of a switch from chlorination to chloramination on microbial communities including Legionella.