Surface charge significantly influences the cellular uptake and cytotoxicity of medical nanoparticles (NPs). This review discusses how the surface charge of NPs affects their interaction with cells, including uptake mechanisms, intracellular localization, and toxic effects. Cationic NPs are more effective in cellular uptake and gene delivery but exhibit higher cytotoxicity compared to anionic NPs. Cationic NPs cause more plasma membrane disruption, mitochondrial and lysosomal damage, and increased autophagosome formation. Nonphagocytic cells generally take up cationic NPs more readily, while phagocytic cells prefer anionic NPs. The uptake routes for cationic and anionic NPs are similar, but higher uptake rates are often linked to greater biological effects. The cytotoxicity of NPs is influenced by factors such as size, shape, composition, surface charge, and hydrophobicity. Cationic NPs are more toxic than neutral or anionic ones, particularly in nonphagocytic cells. Anionic NPs are more toxic in phagocytic cells. Serum can reduce NP toxicity by altering particle surface properties and reducing cellular uptake. The plasma membrane is a primary target for NP-induced toxicity, with cationic NPs causing more membrane damage. NPs can also affect mitochondria, lysosomes, and the nucleus, leading to oxidative stress, DNA damage, and apoptosis. Cationic NPs are more likely to disrupt lysosomes, leading to the release of hydrolytic enzymes and intracellular damage. Anionic NPs may cause less direct damage but can still be toxic through other mechanisms. The surface charge of NPs also influences their intracellular localization and the mechanisms by which they enter cells. Cationic NPs are more likely to be internalized via endocytosis, while anionic NPs may use different pathways. The differences in uptake and toxicity between cationic and anionic NPs are important for the design of NPs for drug delivery and imaging. Understanding these effects is crucial for developing safe and effective NP-based therapies.Surface charge significantly influences the cellular uptake and cytotoxicity of medical nanoparticles (NPs). This review discusses how the surface charge of NPs affects their interaction with cells, including uptake mechanisms, intracellular localization, and toxic effects. Cationic NPs are more effective in cellular uptake and gene delivery but exhibit higher cytotoxicity compared to anionic NPs. Cationic NPs cause more plasma membrane disruption, mitochondrial and lysosomal damage, and increased autophagosome formation. Nonphagocytic cells generally take up cationic NPs more readily, while phagocytic cells prefer anionic NPs. The uptake routes for cationic and anionic NPs are similar, but higher uptake rates are often linked to greater biological effects. The cytotoxicity of NPs is influenced by factors such as size, shape, composition, surface charge, and hydrophobicity. Cationic NPs are more toxic than neutral or anionic ones, particularly in nonphagocytic cells. Anionic NPs are more toxic in phagocytic cells. Serum can reduce NP toxicity by altering particle surface properties and reducing cellular uptake. The plasma membrane is a primary target for NP-induced toxicity, with cationic NPs causing more membrane damage. NPs can also affect mitochondria, lysosomes, and the nucleus, leading to oxidative stress, DNA damage, and apoptosis. Cationic NPs are more likely to disrupt lysosomes, leading to the release of hydrolytic enzymes and intracellular damage. Anionic NPs may cause less direct damage but can still be toxic through other mechanisms. The surface charge of NPs also influences their intracellular localization and the mechanisms by which they enter cells. Cationic NPs are more likely to be internalized via endocytosis, while anionic NPs may use different pathways. The differences in uptake and toxicity between cationic and anionic NPs are important for the design of NPs for drug delivery and imaging. Understanding these effects is crucial for developing safe and effective NP-based therapies.