Objective: To classify intraoperative and postoperative complications using the modified Clavien classification system (MCCS) and modified Satava classification system (SCS) and to evaluate the parameters associated with complications in patients undergoing retrograde intrarenal surgery (RIRS) for renal and proximal ureteral stones. Materials and Methods: We performed a retrospective analysis of 949 patients who underwent RIRS for renal stones and upper ureteral stones at two institutions between March2015 and June2020. Intraoperative complications were assessed using SCC and postoperative complications were graded according to MCCS. Univariate and multivariate analyses were undertaken to determine predictive factors affecting complication rates. Results: The female/male ratio of 949 patients was found to be 346 (36.5)/603(63.5). The median stone size was determined as 13 mm. The stone-free rate was 83.6%after the first intervention, and the final stone-free rate was 94.4% after re-procedure. According to SCS, the intraoperative event and complication incidence was 153(16.1%). MCCS revealed postoperative complications in 121(12.8%) patients. Major complications were observed in 18(1.9%) patients. The rate of complications was higher in patients with renal anomalies (9.9% vs 3.9%, p=0.006). Besides, stone localization, size, number and density were associated with the development of complications (p<0.001, p<0.001, p<0.001 and p=0.002, respectively). In addition, the multivariate analysis revealed that for the patients with grade≥3 complications according to MCCS, only stone-free status was a significant predictor of complication development (p=0.044) whereas for those with grade ≥2b complications according to SCS, significant predictors were stone size (p<0.001), stone density (p=0.022), and fluoroscopy time (p<0.001). Conclusion: This study showed that abnormal kidney anatomy, multiple stones, operative time, and stone-free status were reliable predictors of complication development during and after RIRS. Appropriate preoperative management should be planned according to these predictors to prevent intraoperative and postoperative complications.