The study examined the challenges facing external auditors in Nigeria, focusing on Big Four and indigenous audit firms. A quantitative study design was adopted using secondary data obtained from the annual reports and audited financial statements of 40 selected quoted companies from a population of 161 companies listed on the Nigerian Exchange Group. The study examined auditor independence, technological challenges, workload and time pressure, professional competence, regulatory compliance and audit firm type in relation to audit effectiveness. Data were analysed using descriptive statistics, correlation analysis and multiple regression at a 5% significance level. The regression model produced an R of 0.742, R-square of 0.551 and adjusted R-square of 0.486, indicating that the explanatory variables accounted for 55.1% of the variation in audit effectiveness. The overall model was statistically significant (F = 10.664, p = 0.000). Auditor independence challenges significantly affected audit effectiveness (t = -2.917, p = 0.006), as did technological challenges (t = -2.635, p = 0.013), workload and time pressure (t = -2.459, p = 0.019), professional competence and regulatory compliance (t = -2.554, p = 0.015). Audit firm type also showed a significant difference between Big Four and indigenous firms (t = 2.540, p = 0.016). All five null hypotheses were rejected. The study recommended stronger independence safeguards, increased investment in audit technology, improved staff planning, continuous professional development and stronger regulatory monitoring.