This paper investigates tax specialists’ involvement in both the financial statement audit and non-audit tax services (NATS), and the implications of that involvement for the quality of audit and tax services. Using knowledge sharing theory, we interview audit and tax professionals about their experiences in working with the other service line. First, we identify perceived outcomes that arise from effective collaboration between audit and tax, which go beyond higher quality audit and tax services to include better relationships within the firm and with clients. Second, we find various determinants of tax specialists’ involvement in audit engagements and NATS, specifically complementing prior archival literature by showing interplay of supply and demand factors in contracting for NATS. Unexpectedly, we find significant variation in the nature and extent of specialists’ involvement in the audit of the tax provision. Third, we provide deeper insights about the procedures that tax specialists commonly perform on audit and NATS engagements. Lastly, we provide participants’ insights on factors that contribute to and impede knowledge sharing between auditors and tax specialists. Specifically, we find that various social and behavioral factors, such as personal relationships, attitudes, firm culture, and the tone at the top, are particularly important to effective knowledge sharing. We also find areas of contention between auditors and tax specialists and suggest opportunities for future research to address these factors. Overall, our findings highlight important considerations for practice regarding the relationship between these two service lines and address gaps in the auditing and NATS literatures.