Single nanoparticle detection is of critical importance in many applications. Optical microcavities featuring high-Q factors and small mode volumes have been widely investigated in sensing applications. In general, the microcavity sensing depends mainly on reactive (i.e., dispersive) interactions, resulting in a resonance wavelength shift or mode splitting, which essentially responds to the real part of the polarizability of the targets. In the first part of this talk, we report the experimental demonstration of single nanoparticle detection using either resonance mode broadening or microcavity Raman laser splitting. In the second part, a dissipative sensing method is demonstrated to detect single lossy nanoparticles. This dissipative sensing method holds great potential in detecting nanoparticles of high absorption or ultralow polarizabilities, such as carbon nanotubes and metal nanoparticles, and in characterizing nanoparticle properties in combination with the reactive sensing method.