Ten guidelines in the form of “commandments” are offered to scientists and statisticians who testify before government bodies such as congressional committees, regulatory agencies, or judicial panels. These guidelines are illustrated by examples of testimony by the author and others. Efforts to communicate highly technical material to nontechnical audiences are often frustrated by communication breakdowns. These difficulties can be avoided by making simple statements in plain English, but this isn't so easy as it sounds. The commandments suggest that preparation and hard work are needed to avoid the mistakes that are commonly made in testimony.