This review focuses on the effects of nanoparticle (NP) size on their interactions with live cells, including active and passive cellular internalization and intracellular localization. The article discusses common techniques for characterizing NP size, such as transmission electron microscopy (TEM), dynamic light scattering (DLS), fluorescence correlation spectroscopy (FCS), and nanoparticle tracking analysis (NTA). It highlights how NP size influences the efficiency and kinetics of cellular uptake, the mechanism of internalization, and the subcellular distribution of NPs. The review also explores the cytotoxic effects of NPs, noting that smaller NPs tend to be more toxic due to their larger surface area relative to their mass, which increases the likelihood of adverse chemical reactions. The article emphasizes the importance of understanding NP-size-dependent interactions for designing biocompatible and efficient nanodevices.This review focuses on the effects of nanoparticle (NP) size on their interactions with live cells, including active and passive cellular internalization and intracellular localization. The article discusses common techniques for characterizing NP size, such as transmission electron microscopy (TEM), dynamic light scattering (DLS), fluorescence correlation spectroscopy (FCS), and nanoparticle tracking analysis (NTA). It highlights how NP size influences the efficiency and kinetics of cellular uptake, the mechanism of internalization, and the subcellular distribution of NPs. The review also explores the cytotoxic effects of NPs, noting that smaller NPs tend to be more toxic due to their larger surface area relative to their mass, which increases the likelihood of adverse chemical reactions. The article emphasizes the importance of understanding NP-size-dependent interactions for designing biocompatible and efficient nanodevices.