With the increase in the use of nanotechnology, the release of engineered nanoparticles (NPs) in the environment also increased significantly. Among all the types of NPs available, mainly metal and metal oxide NPs are used widely. They can be used in wide ranges of products like medicines, food additives, cosmetics, sensors, semiconductors. For these reasons, testing their toxicity towards the environment is essential. Because of the availability and easy maintenance in the laboratory, Allium cepa acts as an excellent model system and are used to detect different kinds of environmental pollutants. Cytotoxicity and genotoxicity of different types of metal and metal oxide NPs were tested using A. cepa. Cytotoxicity of NPs was determined by the decrease in percentage mitotic index, which can be further established by estimating the extracellular reactive oxygen species (ROS) production. Measurement of different types of antioxidant enzyme activities could also help us to understand the effects of oxidative stress caused by the ROS. The ROS produced thus can then cause membrane damage by inducing lipid peroxidation, and in turn can enter the cells and damage the genetic materials, i.e., chromosomes. The genotoxicity of NPs towards A. cepa was found out by calculating percentage chromosomal aberrations, which were viewed under optical microscopes as the chromosome size is more prominent in comparison with other plants. Comet assays also confirmed the genotoxicity of NPs in A. cepa root tips. Thus, A. cepa can act as a biomonitoring agent and helps us to analyse the toxic profiles of different NPs, so that proper biosafety measurement can be taken by limiting NPs release in the environment. © 2019 Elsevier B.V.