This paper begins by reviewing the problem of tax evasion measurement. Based on available data, I conclude that this problem is far from being negligible, both for developed and developing countries. I also present in more detail the reasons that justify worrying about tax evasion, and thus the need to study more rigorously this phenomenon. I also revise some of the main results that appear in the theoretical, empirical, and experimental literature of personnel income tax evasion and the theoretical contributions on firms’ tax evasion. Finally, I present policy implications and illustrate them with real case studies.