You must cite the source of all ideas, opinions, facts, and statistics that are not common knowledge. Information that readers in a field would know about from a wide range of general resources is considered common knowledge. For example, in biology, the structure of DNA and the process of cell division or photosynthesis are considered common knowledge. A recent scientific discovery about genetics, however, would not be common knowledge, so you would need to cite the source of this information. The sequence of major historical events is considered common knowledge, but one historian's thoughts about the causes or impact of an event would need to be cited.

If you are uncertain whether your information needs to be cited or not, remember that citing your source properly is never incorrect. When in doubt, the safest course is to use a citation.