Auditors often have prior information about the auditee before starting the substantive testing phase. For example, an auditor might have performed an audit last year, they might have information on certain controls in place, or they might have performed analytical procedures in an earlier stage of the audit. In this article, we show that applying Bayesian statistics in substantive testing allows for integration of this information into the statistical analysis through the prior distribution. This enables auditors to tailor their sampling procedure to the auditee, thereby increasing audit transparency, efficiency, and quality. However, defining a suitable prior distribution can be difficult because what constitutes a suitable prior depends on the specifics of the audit and the auditee. To help the auditor construct a prior distribution we discuss five methodologies, discuss their pros and cons, and give examples of how to apply them in practice.