Nitrous acid (HONO) plays a key role in tropospheric photochemistry, primarily due to its role as a source of hydroxyl (OH) radicals via its rapid photolysis. OH radicals are involved in photooxidation processes, such as the formation of tropospheric O-3 and other secondary atmospheric pollutants (peroxyacetyl nitrate [PAN] and secondary particles). Recent field and modeling studies have postulated the occurrence of a strong and unknown daytime HONO source, but there are still many significant uncertainties concerning the identification and formation mechanisms of these unknown sources. Up to now, five HONO formation pathways are known: direct emission, homogeneous gas-phase reactions, heterogeneous reactions, surface photolysis, and biological processes. In this review paper, the HONO sources proposed to explain the observed HONO budget, especially during daytime, are discussed, highlighting the knowledge gaps that need further investigation. In this framework, it is crucial to have available accurate and reliable measurements of atmospheric HONO concentrations; thus, a short description of HONO measurement techniques currently available is also reported. The techniques are divided into three basic categories: spectroscopic techniques, wet chemical techniques, and off-line methods. [GRAPHICS]