Conventional acoustic remote sensing techniques typically rely on controlled active sources which can be problematic to deploy and operate over the long term, especially if multiple sources are required to fully illuminate the ocean region of interest. Conversely, receiver arrays are becoming increasingly autonomous, miniaturized, and capable of long term deployment thus enabling passive acoustics for ocean remote sensing applications by taking advantage of uncooperative sources of opportunity (e.g., shipping noise) or the ubiquitous ocean ambient noise which have not typically been used in traditional ocean sensing applications. This fully passive approach can also be advantageous when regulations forbid the use of active sound sources or when no active sources are readily available—e.g., at very low frequencies (~10 Hz) or during covert operations. This presentation will discuss the recent development of ocean remote sensing using passive acoustics notably (1) passive acoustic thermometry to estimate deep ocean temperature variations and internal tides using coherent processing of low-frequency ambient noise; (2) localization of drifting sensor networks using ambient noise to enable random volumetric ad hoc receiver array for tracking underwater targets; and (3) monitoring of the shallow water sound channel using shipping sources of opportunity.