The article explores how audit fees impact the quality of audits, specifically focusing on voluntary and mandatory audits in Spanish SMEs. The researchers found that lower audit fees lead to higher quality in voluntary audits, but as fees increase, mandatory audits become more effective at preventing earnings manipulation. They also discovered that abnormal fees in voluntary audits contribute to lower audit quality. Additionally, audit fees only deter earnings manipulation when accruals increase income, which is linked to auditor conservatism.