Silica is a well-known impurity in solid oxide fuel cell raw materials, namely NiO and yttria-stabilized zirconia (YSZ). At elevated temperatures silica will migrate to the grain boundaries, form insulating siliceous phases, and lead to a decrease in the ionic conductivity of the electrolyte. Furthermore, silica impurities have been shown to damage the anode/electrolyte interface, such that an overall decrease in cell performance and long-term stability is observed. Despite the fact that silica is ubiquitous in commercial-grade raw materials and can be incorporated from several extrinsic sources, it has negative effects on the solid oxide fuel cell, such that any further contamination should be avoided to prevent performance degradation and eventual cell failure. This paper reviews and outlines the sources and effects of silica on the solid oxide fuel cell, and attempts to determine a guideline for acceptable levels of silica contamination.