Upconversion nanoparticles (UCNPs) are a promising new generation of imaging agents for bioimaging. They utilize sequential absorption of multiple photons through the use of long lifetime and real ladder-like energy levels of trivalent lanthanide ions embedded in an appropriate inorganic host lattice to produce higher energy anti-Stokes luminescence. This process converts two or more low-energy excitation photons, generally in the near-infrared (NIR) range, into shorter wavelength emissions. UCNPs have unique properties that make them suitable for theranostics, including high resistance to photobleaching, nonblinking behavior, and the ability to penetrate deep into tissues. They are also non-toxic and biocompatible, making them ideal for biomedical applications. The efficiency of UCNPs is influenced by several factors, including the selection of host materials, the tailoring of local crystal fields, plasmonic enhancement, and the suppression of surface-related deactivations. Host materials with low phonon cutoff energy and low crystal field symmetry are preferred to minimize nonradiative losses. The local crystal field can be tailored by doping with nonluminescent cations, which can lower the local symmetry around lanthanide dopants and enhance UC efficiency. Plasmonic enhancement can be achieved by coupling UCNPs with metallic structures, which can significantly increase the UC PL intensity. However, care must be taken to avoid quenching effects by introducing spacer layers between UCNPs and metallic structures. Energy transfer processes within lanthanide dopants are also crucial for UC efficiency. The most efficient ion pairs for UC are Yb³+/Tm³+, Yb³+/Er³+, and Yb³+/Ho³+. The concentration of these ions can be manipulated to enhance UC PL, but higher concentrations can lead to quenching effects. The use of a core/shell structure can help suppress surface-related deactivations and improve UC efficiency. The core/shell structure ensures a homogeneous interface between the core and shell, which enhances the UC PL intensity. UCNPs have a wide range of applications in theranostics, including biosensing, bioassays, high contrast bioimaging, and drug delivery. They can be used for temperature sensing, metal ion sensing, gas molecule sensing, and bioassays. Their ability to penetrate deep into tissues and their high resistance to photobleaching make them suitable for in vivo imaging. UCNPs can also be used for photothermal and photodynamic therapy, as well as for gene and drug delivery. The design and application of UCNPs involve the use of nanochemistry to control their optical properties and enhance their efficiency. The synthesis of UCNPs can be achieved through various strategies, including thermolysis, Ostwald-ripening, and hydro(solvo)thermal methods. Surface engineering techniques, such as ligand exchange, ligand removal, and layer-by-layer assembly, can be used to modifyUpconversion nanoparticles (UCNPs) are a promising new generation of imaging agents for bioimaging. They utilize sequential absorption of multiple photons through the use of long lifetime and real ladder-like energy levels of trivalent lanthanide ions embedded in an appropriate inorganic host lattice to produce higher energy anti-Stokes luminescence. This process converts two or more low-energy excitation photons, generally in the near-infrared (NIR) range, into shorter wavelength emissions. UCNPs have unique properties that make them suitable for theranostics, including high resistance to photobleaching, nonblinking behavior, and the ability to penetrate deep into tissues. They are also non-toxic and biocompatible, making them ideal for biomedical applications. The efficiency of UCNPs is influenced by several factors, including the selection of host materials, the tailoring of local crystal fields, plasmonic enhancement, and the suppression of surface-related deactivations. Host materials with low phonon cutoff energy and low crystal field symmetry are preferred to minimize nonradiative losses. The local crystal field can be tailored by doping with nonluminescent cations, which can lower the local symmetry around lanthanide dopants and enhance UC efficiency. Plasmonic enhancement can be achieved by coupling UCNPs with metallic structures, which can significantly increase the UC PL intensity. However, care must be taken to avoid quenching effects by introducing spacer layers between UCNPs and metallic structures. Energy transfer processes within lanthanide dopants are also crucial for UC efficiency. The most efficient ion pairs for UC are Yb³+/Tm³+, Yb³+/Er³+, and Yb³+/Ho³+. The concentration of these ions can be manipulated to enhance UC PL, but higher concentrations can lead to quenching effects. The use of a core/shell structure can help suppress surface-related deactivations and improve UC efficiency. The core/shell structure ensures a homogeneous interface between the core and shell, which enhances the UC PL intensity. UCNPs have a wide range of applications in theranostics, including biosensing, bioassays, high contrast bioimaging, and drug delivery. They can be used for temperature sensing, metal ion sensing, gas molecule sensing, and bioassays. Their ability to penetrate deep into tissues and their high resistance to photobleaching make them suitable for in vivo imaging. UCNPs can also be used for photothermal and photodynamic therapy, as well as for gene and drug delivery. The design and application of UCNPs involve the use of nanochemistry to control their optical properties and enhance their efficiency. The synthesis of UCNPs can be achieved through various strategies, including thermolysis, Ostwald-ripening, and hydro(solvo)thermal methods. Surface engineering techniques, such as ligand exchange, ligand removal, and layer-by-layer assembly, can be used to modify