The adoption of audit software and related technologies is one of the major challenges facing the audit profession in the digital age. Despite the obvious benefits of these technologies, including increased efficiency, accuracy, and audit quality, many audit firms, especially small firms, face numerous barriers to their adoption and use. This qualitative study was conducted with the aim of identifying, categorizing, and comparing barriers to adoption of audit software in small and large firms. The statistical population included audit firm partners and financial managers of companies listed on the Tehran Stock Exchange, and data were collected through semi-structured interviews and questionnaires. The findings show that barriers to adoption fall into three main categories: financial and economic barriers (including implementation costs, return on investment, and lack of economic justification), knowledge and skill barriers (including lack of technical expertise, resistance to change, and lack of understanding of benefits), and organizational and infrastructure barriers (including weak IT infrastructure, lack of standardization, and inadequate support). The results show that financial barriers are more important for small institutions and knowledge and infrastructure barriers are more important for large institutions.