Theranostics combines diagnostics and therapy in a single multifunctional system. Multifunctional upconversion luminescent lanthanide-doped nanothermometers for theranostic purposes offer non-invasive and sensitive multimodal performance in the biomedical field over traditional temperature measurement methods. Despite existing challenges, various studies on hybrid upconversion nanothermometers show substantial progress for (bio)imaging, temperature sensing, photodynamic and photothermal therapy, as well as drug delivery applications. The beauty of such an approach is that it unfolds possibilities to combine diagnostics and therapy in a single particle, which can modify the way certain diseases are treated, hence change the entire healthcare scene.