Theory from organizations and economics research posits that in an inter-organizational relationship, both parties invest in relationship-specific knowledge, which in turn facilitates the effectiveness of the relationship while strengthening the attachment between the parties. In complex settings where there are more opportunities for knowledge creation, the investments will be larger and the attachment stronger. Because banks are complex institutions that present unique challenges to auditors, we suggest that effective audits critically depend on the accumulation of significant investments in client-specific expertise through a long association with the client. We find a positive association between audit firm tenure and financial reporting quality, and this association is particularly strong in banks that are more complex. Also, contrary to recent research we find that benefits of audit firm tenure for complex banks accrue even for long tenure and are not limited to medium tenure. Our findings largely support the notion that a long relationship with the client reflects the underlying demand for expertise, which is critical for high-quality audits of complex organizations. Imposing short-term limits on audit firms would adversely affect the investments in client-specific expertise especially in the cases where this expertise is needed the most. Our findings do not support calls for mandatory audit firm rotation for large complex institutions.