Semi-volatile organic compounds (SVOCs) are defined as organic compounds that show significant gas and particulate concentrations in the atmosphere. Nearly all classes of organic compounds contain semi-volatiles species: alkanes, polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB), nitro-aromatics, terpenes, acids, carbonyls, and lipids, to name a few. SVOCs enter the atmosphere by direct emission, frequently as byproducts of incomplete combustion. Polar SVOCs are also produced by oxidation of precursor unsaturated compounds, and they can be incorporated into ambient particulate matter (PM) as secondary organic aerosols (SOA). Precursor organics are emitted from transportation, industrial and biogenic sources. Also, many of the potentially carcinogenic organic compounds found in the atmosphere are semi-volatile. The degree of gas-particle partitioning affects the transport, deposition, and atmospheric fate of these compounds, since SVOCs, once airborne, can deposit onto vegetative surfaces, windows, carpets, soils, or even the human body. Compounds with higher vapor pressures are present primarily in the gas phase, whereas low volatility compounds are found on or within particles. SVOCs and PM-associated organic carbon (OC) are related through the partitioning of SVOCs onto PM. Hundreds of individual organic compounds have been identified in the organic atmospheric aerosol so far; however, together they constitute less than 10% of the OC of urban and rural aerosol. A large and until recently unaccounted fraction of the continental organic aerosol consists of polymeric or oligomeric substances. In mass closure studies on organic aerosol “humic-like substances (HULIS)”, or “organic macromolecules”, or “polycarboxylic acids”, are generally claimed to resolve a large fraction of unexplained mass of particulate organic carbon. This presentation will discuss the challenges encountered in sampling and characterization of atmospheric SVOCs. In addition, the unresolved and unexplained mass of ambient PM and the new analytical method employed for its characterization will be briefly discussed.