Lanthanide-doped upconversion nanoparticles (UCNPs), such as NaYF4 doped with Yb3+ and Er3+, exhibit narrow and high-intensive emissions in the visible spectrum when excited by a low power energy, continuouswave (CW) near-infrared (NIR) laser, that is, a large anti-Stokes shift (see Figure  5.1). Of special relevance for biomedical applications, NIR light exhibits a remarkably deep penetration into tissues, and UCNPs present luminescence with a high signal-to-noise ratio, they are very resistant to photobleaching and photoblinking, they exhibit excellent chemical and thermal stability and relatively low toxicity, and they are suitable for long-time particle tracking (G. Chen et al. 2014; Dacosta et al. 2014; Gnach et al. 2015; Gu et al. 2013; Y. Liu et al. 2013; M. Wang et al. 2011). These unique properties not only make UCNPs particularly ideal in nanomedicine, but they are also promising in nanotechnology, since they can be used for photovoltaic cells (as NIR absorbers), photocatalysis (as sensitizers), and security applications (anti-counterfeiting materials) (G. Chen et al. 2015; Gnach and Bednarkiewicz 2012; Ramasamy et al. 2014; Won Jin et al. 2009).