Engineered carbon nanotubes (CNTs) are revolutionizing the field of biotechnology, with advances in medicine, water purification, construction, and military applications because of their unique properties. With many products on currently on the market containing CNTs, contact with the aquatic environment is inevitable throughout the life cycle of CNT-based products. The U.S. Environmental Protection Agency has listed CNTs and other nanomaterials as Contaminants of Emerging Concern; however, the lack of information available, specifically on toxicity to aquatic organisms, hinders the EPA from moving forward with regulations. The purpose of this dissertation was to provide data on chronic exposures of 14C-labeled multi-walled carbon nanotubes (MWCNTs) to model organisms typically used in traditional toxicity tests and determine how the presence of dissolved natural organic matter (NOM) influenced the reproductive toxicity of MWCNTs and overall organism health. We observed that exposure to 2.5 mg/L sonicated MWCNTs decreased the number of offspring released and brood size in Ceriodaphnia dubia, but the presence of NOM (2.35 mg C/L) negated this effect, but at a cost of increased internal accumulation. The presence of NOM also influenced toxicity of MWCNTs to Americamysis bahia, where a decrease in the percentage of mature individuals was observed with exposure. We also exposed Pimpephales promelas to 14C-MWCNTs-contaminated sediment. P. promelas did not accumulate measurable MWCNTs, but we hypothesize that a 24-h depuration period is sufficient for them to clear their gut tract, as measurable MWCNTs were collected in fecal material released during the depuration period. We also observed that bioturbation of sediment by P. promelas can influence the ability of MWCNTs to move into the water column but access to the sediment did not change the incidence of morbidity. Finally, we analyzed the literature available on crustacean toxicity after exposure to carbon nanoparticles, integrating my observations from previous studies, to predict mode(s) of action of MWCNTs toxicity to aquatic organisms and give guidance for future research. Overall, these findings provide some of the data needed for a successful risk assessment framework for future regulation of MWCNTs and highlight the importance of NOM in understanding the toxicity of MWCNTs.