Rare-earth doped nanoparticles (NPs) are known to have exceptional optical properties such as strong absorption and sharp emission lines. Since the optical properties are due to the f-f electron transitions, these nanoparticles do not photobleach like organic dyes or blink like quantum dots. Through the process known as upconversion, rare-earth ions can convert multiple lower energy photons into higher energy ones. We utilize this to engineer our NPs to absorb near-infrared light (980 nm) and emit in the visible (550 nm, 660 nm) and NIR (800 nm). Also, these NPs do not require high energy densities such as conventional two-photon processes and can be excited with pulsed and continuous sources, making them ideal for biomedical purposes. The 980 nm excitation for ytterbium and erbium co-doped systems, such as the KMgF3: Yb/Er and NaGdF4: Yb/Er NPs used herein, is advantageous due to the low scattering and absorption of tissue in this region.