Neonicotinoid insecticides are a group of water-soluble pesticides commonly used in agriculture. Their transportation to nearby waterways presents the potential for toxicity to non-target aquatic invertebrates—especially aquatic insects. The lack of toxicity data for developing water quality guidelines has been highlighted as a key knowledge gap by jurisdictions including Canada. The objective of this thesis, therefore, was to generate toxicity data for six neonicotinoids (acetamiprid, clothianidin, dinotefuran, imidacloprid, thiacloprid, thiamethoxam) for a suite of aquatic invertebrates under different exposure scenarios: acute (2- to 4-day), pulse (24-hour, then chronic recovery), and chronic (7- to 56-day). These data were used to evaluate the potential hazard of neonicotinoids to aquatic invertebrates in a southern Ontario context. As neonicotinoids are commonly applied with other pesticides, toxicity of imidacloprid was also examined in a binary mixture with the fungicide, tebuconazole. Acute toxicity data generated for ≥ 20 aquatic invertebrates showed the laboratory-cultured insects Chironomus dilutus and Neocloeon triangulifer were generally most sensitive to neonicotinoids, with LC50s < 12 µg L-1. Hazard assessment showed a moderate acute hazard to aquatic invertebrates for imidacloprid; for other neonicotinoids, no or low hazard was concluded. Single-pulse exposures showed C. dilutus and N. triangulifer were immobilized after 24-hour exposure to 9 µg L-1 imidacloprid but recovered when transferred to clean water with no long-term effects on emergence. Chronic toxicity data for C. dilutus and N. triangulifer showed effects on survival, growth, and emergence at concentrations < 2 μg L-1 for most neonicotinoids. Ceriodaphnia dubia was several orders of magnitude less sensitive compared to insects. Chronic hazard assessment concluded a moderate hazard to aquatic invertebrates for clothianidin, and a high hazard for imidacloprid. Binary mixtures of imidacloprid and tebuconazole showed no conclusive synergistic effects; only effects on Hyalella azteca deviated from additivity (independent action model) and were dose-ratio antagonistic. Tebuconazole was less potent than imidacloprid; when combined with the lack of synergistic effects, it was concluded that tebuconazole does not pose a greater acute hazard to aquatic invertebrates than imidacloprid alone. Data presented in this thesis will contribute to developing water quality guidelines and risk assessments in Canada and abroad.