Perchlorate is a widespread environmental pollutant and a known thyroid hormone disruptor. Little is known about the sources and pathways of human exposure to perchlorate. We measured perchlorate in drinking water, foodstuffs, indoor dust and urine collected from various locations in the United States, with a vast majority of them from New York State. The highest concentration of perchlorate was found in indoor dust, at a geometric mean (GM) concentration of 280 ng/g, followed by foodstuffs including beverages (1.77 ng/g) and drinking water (0.09 ng/mL). Among nine categories of foodstuffs analyzed, perchlorate concentrations as high as 839 ng/g were found in vegetables. The estimated average perchlorate exposure dose for adults from various sources was 141 ng/kg body weight (BW)/day. Previously reported perchlorate concentrations in breast milk from the United States in 2007 were used for the estimation of exposure doses in infants, and were 1580, 1230, and 1010 ng/kg BW/day for 0-3, 3-6, and 6-12 months of age, all exceeded the United States Environmental Protection Agency's (EPA) chronic reference dose (700 ng/kg BW/day). Breastfeeding was the dominant exposure pathway accounting for > 90% of perchlorate exposure among infants < 1 year, whereas milk, vegetables, and beverages were the dominant exposure sources (73-83%) for age groups > 1 year. Perchlorate was detected in all human urine collected from Albany, New York (N = 31) at a GM concentration of 2.30 ng/mL. The GM perchlorate exposure doses calculated from urinary concentrations, through a reverse dosimetry approach, were 46.1 and 37.8 ng/kg BW/day for age groups 16-21 and > 21 years, respectively, which were similar to those calculated from external sources of exposure.