Referring to sources is a vital part of scientific communication. There could be a wide range of relevant sources within your field of study and so you need to assess whether they are useable for academic writing.
It is sensible to find earlier research that may support your argument for a certain viewpoint. These sources should be of sufficient quality.
The advice on this page is meant as a guide in the process of assessing sources for writing at university-level. Which sources are suitable for your essay would be dependent on the context, and must be assessed individually. Nevertheless, there are some general rules you should be aware of whenever you assess a source.