Non-parametric tests, in particular rank-based tests, are often proposed as robust alternatives to parametric tests like t-tests when the assumptions of parametric tests are violated. However, non-parametric tests have their own assumptions which, when not considered, can lead to misinterpretation and unsound conclusions based on those tests. This chapter explores these problems and differentiates between the more and less robust non-parametric tests. Modern robust alternative non-parametric tests are suggested to replace the less robust tests.