Optical imaging represents one of the most essential tools in biological studies. Although with great advances, bio-optical imaging still suffers from problems such as resolution, sensitivity, speed, and penetration depth. Due to the unique optical properties of gold nanoparticles (AuNPs), i.e., surface plasmon resonance, AuNPs can be readily used to enhance optical imaging based on their absorption, scattering, fluorescence, Raman scattering, etc. Here, we include the most recent achievements and challenges associated with using AuNPs to improve resolution and sensitivity in biological imaging in vitro and in vivo. The application of AuNPs in the following three aspects were discussed: 1) Direct visualization of AuNPs inside the biosystems using i) dark field (DF) microscopy, ii) differential interference contrast (DIC) microscopy, and iii) other techniques, such as interferometric scattering (iSCAT) microscopy and photothermal imaging. Additionally, since orientation and rotational motions are closely related to various biological processes, we also summarized the recent advances of optical imaging methods in the rotational and orientation tracking of AuNPs. 2) Monitoring of biomolecular events and physiological processes using i) surface-enhanced Raman spectroscopy (SERS) and ii) plasmon enhanced fluorescence (PEF) for ultra-sensitive detection of biomolecules, including proteins, metabolites, DNA, RNA, etc. 3) In vivo deep tissue imaging using i) two-photon and/or multi-photon imaging, ii) optical coherence tomography (OCT), and iii) photoacoustic (PA) imaging for disease diagnoses, such as detecting tumors and other diseases in eye, brain, and bone. In conclusion, based on our literature study, AuNPs-assisted bioimaging acts as a promising tool in exploring fundamental biological questions and early diagnosis of diseases. (C) 2019 Elsevier Ltd. All rights reserved.