A systematic analytical investigation has been performed by The Boeing Aerospace Company under the sponsorship of Air Force Rome Air Development Center to identify molecular and particulate contaminants that affect the optical sensor of a space-based surveillance system during prelaunch, launch, deployment, and lifetime operation of the system. This paper reports results of this investigation and includes discussion on contamination sources and species, contaminant transport and deposition mechanisms, and their impact on optical sensors in the ultraviolet, visible, and infrared regimes. The effects of cryodeposits on infrared sensor surfaces and organic deposition on ultraviolet sensors are addressed. Methods of contamination prevention and control are also summarized. The results of this investigation show that contamination of optical sensors can reduce or eliminate a sensor's ability to detect and identify targets. This performance degradation can occur directly or indirectly as a result of contamination, depending on the system design. Some effects of contamination include stray light scattered into the field-of-view, false targeting on particles, attenuation of signal, and warming of optical train surfaces due to increased absorptance of contaminated radiators and other surfaces. These effects are categorized and presented in the form of molecular/particulate deposition and free molecules/particles in the field-of-view.