Why do we find statistical considerations, such as outlier tests or validation, in a book on the practice of immunoassays? The answer is quite simple: immunoassays are supposed to provide us with results. They are a tool for making statements, e.g., in diagnostics. But can we really trust the values measured and their interpretation? The attempt to answer the question of trust is called validation. A validation should thus show that the binding events of the assay reagents correlate with the presence (or absence) of the analyte. This chapter provides an overview and guidance on how to validate immunoassays.