We conducted a systematic review of 124 published articles that investigated the toxicity of microplastics and nanoplastics to Daphnia magna. This review summarizes studies assessing acute, chronic, and multigenerational impacts, as well as the effects observed via leached chemicals from plastics and the role of plastics as contaminant vectors. Overall, observed toxicity varies across different polymer types, and shapes. One of the most visible findings is that targeted research synthesis of the acute toxicity tests found more toxicity in smaller-sized particles. Most studies use spherical plastics that are commercially available, especially polystyrene, while the use of irregular-shaped and/or secondary plastics is still emerging. Also, there are still various confounding factors that make the comparison of the observed results difficult. Future studies should focus on irregular-shaped particles, and other polymer types, besides polystyrene. More research efforts are needed to understand the impacts of environmental factors and complex matrices.