One of the major concerns in any Cosmic Microwave Background (CMB) anisotropy analysis is to determine which fraction of the observed signal is due to some foreground contaminant. Two sources of foreground contamination have been firmly identified: the diffuse Galactic emission and unresolved point sources.
The diffuse Galactic emission includes three components: synchrotron and free-free radiation, which are important mainly at frequencies below 60 GHz, and thermal emission from dust particles, which is important mainly at frequencies above 60 GHz. From the theoretical point of view, it is possible to distinguish these three components by observing their different frequency dependence and spatial morphology. In practice, however, there is no emission component for which both the frequency dependence and spatial template are currently well known. Besides the above cited mechanisms, recent evidence suggests that there are additional physical components that can contaminate the CMB signal.