Summary Polybrominated diphenyl ethers (PBDEs) are a class of brominated flame retardants. The addition of these chemicals to electrical and electronic equipment, building materials, carpet and textiles reduces flammability and therefore harm and destruction caused by unwanted fire. These chemicals are now internationally ubiquitous in the environment and humans because of their increased usage and physical and chemical properties of persistence and lipophilicity. Although Australian data on both the concentrations and the sources and exposure pathways was limited, a preliminary study of PBDEs in 10 pools of human blood serum revealed that concentrations of PBDEs in Australia were comparatively higher than most countries with the exception of Canada and the USA. The aim of this project was to determine the concentrations of PBDEs in the Australian population and to examine the possible sources and pathways of exposure in humans. Experimental evaluation of PBDE exposure matrices included samples of: sediment (n=46), air (n=19), dust (n=19), surface wipes (n=10) and human milk (157 samples combined into 17 pools, n=10 individual samples); and the general population using human blood serum (10552 samples combined into 169 pools). With accumulating evidence regarding age differences in PBDE concentrations, the scope of this project was broadened to include a focus on infants and young children. This study found that PBDE contamination was widespread in the Australian environment and human population. In sediment, PBDEs were relatively low at the majority of sites with mean ± standard deviation and median ΣPBDE concentrations across all sites of 4707 ± 12580 and 305 pg g-1 dw, respectively. There were elevated concentrations found downstream of sewerage treatment plants; in areas dominated by industrial and urban land-use types; and in estuarine compared to freshwater environments. PBDEs were detected in air, dust and surface wipes. Concentrations on the whole were found to be higher indoors compared to outdoors and in offices compared to homes. ΣPBDE concentrations ranged from 0.5 -179 pg/m3 for homes and 15 - 487 pg/m3 for offices and at the two outdoor sites the concentrations were 1.7 and 6.8 pg/m3. PBDEs were detected on 9 out of 10 surfaces (e.g. television, refrigerator, DVD player) sampled and ranged from non-detectable to 5985 pg/cm2 while in dust, ΣPBDEs ranged from 87 - 733 ng/g dust in homes and 583 - 3070 ng/g dust in offices. PBDEs were detected in all individual and pooled samples of human milk and blood serum and the ΣPBDE and BDE-47 concentrations ranged from 5.5 to 103 and 0.6 to 55 ng/g lipid, respectively. There were no regional differences in concentrations in human milk or blood serum and in general PBDE concentrations were higher in males than in females. Although no temporal trend was apparent in samples of human milk or blood serum collected between 2002 and 2008, the ban on penta- and octa-BDE commercial products in several countries has only been in place since 2004. Hence, effects of the ban may not be reflected in human body burden for several years. The concentrations of PBDEs were found to be highest in young children aged 2 – 5 years of age where concentrations were up to 10 times greater than those detected in adults. Congener profiles for sediment, air, dust and surface wipes were mostly dominated by BDE-209. In human milk and serum, BDE-47 was dominant while BDE-209 contributed only 3% in human milk and was not determined in serum. Concentrations of PBDEs found in Australian human and environmental samples were lower than those reported from North America but higher than those reported from Europe and Asia. The results of modeling and the assessment of matched samples of human milk, indoor air and dust showed that body burden could be partly explained by exposure via food, air, dust and human milk as well as clearance (half-life) data. However, the higher than expected concentrations in young children indicated that there are likely to be missing sources and exposure pathways and/ or the clearance data is inaccurate. The results of this thesis provide baseline data on PBDE concentrations in Australia and will prove important for long term monitoring of the effect of changes in usage rates of these chemicals. It is important to understand the specific sources and exposure pathways of PBDEs in infants and young children in order to identify intervention methods whereby exposure to this vulnerable population can be reduced. In terms of the future use of PBDEs or other flame retardant chemicals, it is important that the risk-benefit ratio is regulated so that the minimization of human exposure and potential associated negative health effects is balanced with the reduction of harm caused by fire.