I have quantified the energy requirements of leatherback turtles (Dermochelys coriacea) throughout development, and examined growth rates, resource requirement and availability, and anthropogenic threats from the commercial fishery. I demonstrated that the use of the doubly labeled water (DLW) method to determine field metabolic rate in marine turtles is constrained by low metabolic (MR) and high water turnover rates (Chapter 2). For fed and fasted turtles, water turnover rates were 9.57±1.33% and 6.14±0.65% TBW day−¹1 and MR (from respirometry) was 28.66±5.31 kJ kg−¹ day−¹ and 13.77±1.49 kJ kg−¹ day−¹, respectively. This led to isotope turnover (kd:ko) ratios of 0.91±0.02 for fed turtles and 1.08±0.16 for fasted turtles, producing negative MRs for fasted turtles. While I showed that for fed turtles the DLW method was consistent with respirometry the use of DLW in fasting turtles differed from respirometry by 440%. The fact that DLW does not work in certain situations is a rare finding that will be of broad interest in the field of energetics. Having determined that the DLW method is constrained in marine turtles I then turned to rearing leatherbacks in the laboratory to measure growth (Chapter 3) and determine energy intake (Chapter 4). For the first time I was able to rear several leatherbacks from hatching to juveniles. Leatherbacks maintained an average growth rate of 31.9 ± 2.8 cm year−¹ in straight carapace length (SCL) throughout the study period. The captive leatherbacks matched the length-mass relationship of wild juveniles and adults. A von Bertalanffy growth function (VBGF) predicted age-at-maturity for leatherbacks of 15.3 years. Bycatch data, supplemented with growth curve data, indicate that leatherbacks will reach the minimum length at which they are found interacting with fisheries (drift gill net and longline) in less than 3 years, suggesting they are exposed to threats from marine fisheries for > 80 % of their life before maturity is attained. I estimated that the majority of the Pacific Ocean population of leatherbacks is made up of 2-6 year old juveniles (137,368 turtles) consuming 1.6 x 10⁶ tonnes of jellyfish year−¹. These turtles are restricted to warmer equatorial waters where primary productivity and, possibly, jellyfish abundance are low.