Incorporation of nanoparticles in daily products and biomedical devices has raised concern regarding their potential toxic effects on human health. This leads to the emergence of nanotoxicity, a field that studies and addresses the toxicity of nanoparticles. Both in vitro and in vivo studies have identified reactive oxygen species as one of the primary causes of nanotoxicity. The presence of nanoparticles in cells also leads to an activation of inflammatory response and autophagy as they are recognized as foreign bodies. Ultimately, these molecular events lead to cell death via apoptosis. More recently, nanoparticles are discovered to interact with the constituents of a cell, leading to organelle dysfunctions, corona effect, protein citrullination, and catabolism induction. Understanding the molecular mechanisms underlying nanotoxicity is the first step to devising possible strategies to circumvent nanotoxicity, rendering them safer for use. In this chapter, a comprehensive summary of molecular mechanisms underlying nanotoxicity and its repercussions is described.