Understanding the interactions of nanoparticles (NPs) with cells and how these interactions influence their cellular uptake is essential to exploring the biomedical applications of NPs, particularly for drug delivery. Various factors, whether differences in physical properties of NPs or variations in cell-membrane characteristics, influence NP-cell interactions and uptake processes. NP-cell membrane interactions may also influence intracellular trafficking of NPs, their sorting into different intracellular compartments, cellular retention, and hence the efficacy of encapsulated therapeutics. A crucial consideration is whether such interactions might cause any toxicity, starting with how NPs interact in transit with the biological environment prior to their interactions with targeted cells and tissues. Understanding the effects of various NP characteristics on cellular and biological processes could help in designing NPs that are efficient but also nontoxic.