This chapter discusses evaluating the reliability of knowledge and the credibility of sources with the overall aim of identifying and using strong evidence as one of the key elements of critical thinking. Evidence is something tangible that determines your line of reasoning and supports your argument to make it more convincing. Sources need to be evaluated for their credibility based on expertise and vested interest, but you should also use independent verification through gatekeepers who assess the underlying knowledge. You can place various sources on a continuum from least to most credible, including blogs; agenda-driven news media; company websites; Wikipedia; consultants, think tanks, and NGO reports; respected and established news media; reputable organization reports; audited reports; and academic sources. To find academic sources of evidence, you should identify the leading textbook in your field and learn to use an academic journal database to identify key articles. Referencing is a standardized way of demonstrating your knowledge of the topic, demonstrating the credibility of your evidence, providing the reader with avenues to explore ideas further, and ensuring good academic practice by acknowledging the work of others and avoiding plagiarism.