This review discusses the cellular uptake of nanoparticles (NPs) and the factors influencing their interactions with cells. NPs can modulate cell fate, induce or prevent mutations, and influence cell-cell communication. Recent advances in chemical synthesis have enabled the creation of NPs with precisely defined biochemical features, while emerging analytical techniques have provided insights into nano-bio interactions within cells. The review covers the journey of NPs inside cells, focusing on both extracellular and intracellular nano-bio interactions. The cell membrane (CM) protects intracellular components and regulates the entry and exit of small molecules and nutrients. NPs can interact with the CM through various mechanisms, including phagocytosis, clathrin-mediated endocytosis (CME), caveolae-mediated endocytosis, clathrin/caveolae-independent endocytosis, and macropinocytosis. Phagocytosis involves the engulfment of NPs by professional phagocytes, often facilitated by opsonization. CME is the main mechanism for nutrient and membrane component uptake, while caveolae-mediated endocytosis is involved in various biological processes. Clathrin/caveolae-independent endocytosis occurs in cells lacking these structures, and macropinocytosis involves the formation of large membrane extensions that fuse back onto the plasma membrane. The physicochemical properties of NPs, including size, shape, surface charge, and hydrophobicity, significantly influence their cellular uptake. Larger NPs are more efficiently taken up by phagocytes, while shape affects uptake rates. Surface charge influences NP interactions with the CM, with cationic NPs showing higher uptake due to electrostatic interactions. Hydrophobicity also plays a role, with hydrophobic NPs stabilizing around the membrane core and semi-hydrophilic NPs adsorbing to the bilayer surface. The microenvironment around target cells, including pH and the presence of fibrosis, can alter NP properties and affect their interactions with the CM and intracellular fate. For example, in the tumor microenvironment, high pressures can restrict NP extravasation, while low pH can alter NP surface charge, enhancing tumor cell entry. The design of NPs for specific applications must consider the microenvironment of target cells to optimize their performance and minimize toxicity. Understanding the mechanisms of NP uptake and trafficking is crucial for designing efficient and safe nanomedicines. The review highlights the importance of tuning NP physicochemical properties to optimize cellular targeting, uptake, and trafficking. The interplay between NP properties and the cellular microenvironment determines the success of NP-based therapies.This review discusses the cellular uptake of nanoparticles (NPs) and the factors influencing their interactions with cells. NPs can modulate cell fate, induce or prevent mutations, and influence cell-cell communication. Recent advances in chemical synthesis have enabled the creation of NPs with precisely defined biochemical features, while emerging analytical techniques have provided insights into nano-bio interactions within cells. The review covers the journey of NPs inside cells, focusing on both extracellular and intracellular nano-bio interactions. The cell membrane (CM) protects intracellular components and regulates the entry and exit of small molecules and nutrients. NPs can interact with the CM through various mechanisms, including phagocytosis, clathrin-mediated endocytosis (CME), caveolae-mediated endocytosis, clathrin/caveolae-independent endocytosis, and macropinocytosis. Phagocytosis involves the engulfment of NPs by professional phagocytes, often facilitated by opsonization. CME is the main mechanism for nutrient and membrane component uptake, while caveolae-mediated endocytosis is involved in various biological processes. Clathrin/caveolae-independent endocytosis occurs in cells lacking these structures, and macropinocytosis involves the formation of large membrane extensions that fuse back onto the plasma membrane. The physicochemical properties of NPs, including size, shape, surface charge, and hydrophobicity, significantly influence their cellular uptake. Larger NPs are more efficiently taken up by phagocytes, while shape affects uptake rates. Surface charge influences NP interactions with the CM, with cationic NPs showing higher uptake due to electrostatic interactions. Hydrophobicity also plays a role, with hydrophobic NPs stabilizing around the membrane core and semi-hydrophilic NPs adsorbing to the bilayer surface. The microenvironment around target cells, including pH and the presence of fibrosis, can alter NP properties and affect their interactions with the CM and intracellular fate. For example, in the tumor microenvironment, high pressures can restrict NP extravasation, while low pH can alter NP surface charge, enhancing tumor cell entry. The design of NPs for specific applications must consider the microenvironment of target cells to optimize their performance and minimize toxicity. Understanding the mechanisms of NP uptake and trafficking is crucial for designing efficient and safe nanomedicines. The review highlights the importance of tuning NP physicochemical properties to optimize cellular targeting, uptake, and trafficking. The interplay between NP properties and the cellular microenvironment determines the success of NP-based therapies.