Nanocrystals with second harmonic response is a new class of nonlinear optical nanoprobes with dramatically different properties from fluorescent agents. Compared with two-photon fluorescence, second harmonic generation is an ultrafast, lossless, and coherent process. In particular, the absence of photobleaching and emission intermittency in the optical response of the second harmonic nanoparticles is likely to complement the fluorescent agents widely used today in many imaging applications. Furthermore, the coherent emission from the second harmonic generation process provides unique opportunities for the application of coherence domain techniques that are not available with fluorescent agents. We review the application of the second harmonic nanocrystals in imaging applications, especially those pertaining to biomedicine.