Upconverting nanoparticles (UCNPs) doped with lanthanide ions convert near-infrared radiation into visible emission through sequential excitation processes. While the fundamental mechanisms of upconversion are well established, recent studies reveal less intuitive photophysical behaviors in these systems. In this seminar, I will highlight selected examples of such phenomena. In particular, I will discuss a pronounced enhancement of upconverted emission under dual-wavelength near-infrared co-excitation, where simultaneous excitation of ground- and excited-state transitions produces emission intensities far exceeding those obtained with single-wavelength excitation. I will also briefly address upconversion superfluorescence, where ensembles of emitters synchronize to generate intense, short bursts of radiation. These examples demonstrate how the complex energy-level structure and long lifetimes of lanthanide ions give rise to unconventional optical dynamics with potential applications in nanophotonics and imaging.