Aim: The aim of the doctoral thesis was to examine, ex vivo, the effectiveness of a novel laseractivated photoacoustic streaming technique, SWEEPS (Shock Wave Enhanced Emission Photoacoustic Streaming), in root canal retreatment and against bacterial biofilm, and to compare it with Er:YAG laser activated SSP modality (LAI/SSP), ultrasonically activated irrigation (UAI) and conventional syringe needle irrigation (SNI) techniques. Additionally, the effectiveness of the SWEEPS was evaluated by using two different laser tips: flat and radial. Material and methods: In the first part of the doctoral thesis, the SWEEPS technique was evaluated in the removal of root canal filling remnants from severely curved root canals after a conventional root canal retreatment. The study sample consisted of 40 severely curved root canals of extracted human upper molars. The canals were instrumented and filled with an epoxy resin-based sealer and gutta-percha using a continuous wave vertical compaction and a warm injection back-filling. Root canal retreatment was performed using ProTaper Universal Retreatment files without solvent; instead, only sodium hypochlorite (NaOCl) was used. Following the preparation, all samples were randomly divided into four groups (n = 10) according to the irrigation technique: UAI, LAI/SSP, SWEEPS/flat-tip, and SWEEPS/radialtip, using 6 ml of 3 % NaOCl for an activation time of 90 seconds, divided into 3 cycles of 30 seconds. All tested samples were subjected to three micro-CT scans: after the root canal filling, after the root canal retreatment, and after the final irrigation in order to calculate the volume of the filling material in root canals. In the second part of the study, the retreatment efficacy of the SWEEPS was evaluated in oval root canals filled with a bioceramic based sealer. Total of 42 extracted human mandibular molars with oval distal root canals were included in the study. The samples were prepared up to size 40/0.06 and filled with a bioceramic sealer in combination with a single cone obturation technique. The canals were retreated using Reciproc Blue RB40 files and 3 % sodium hypochlorite solution, without solvent as well. The samples were then randomly divided into three groups (n = 14): SWEEPS/radial tip, UAI, SNI; and final irrigation protocols were performed. Micro-CT scans were performed at every stage of the root canal retreatment as in the first part of the study. Evaluation of the antimicrobial effectiveness of SWEEPS technique was conducted against mature Enterococcus faecalis (E.faecalis) biofilm grown on dentine discs for 14 days, after which the samples were randomly assigned to the tested groups with control samples in each group: SWEEPS/radial tip, UAI, SNI. Each group consisted of positive control samples, samples for serial dillution and a culture method, and samples for a qualitative analysis using confocal laser microscopy (CFLM) and scanning electron microscopy (SEM). Each sample was irrigated using 3 % NaOCl for 90 seconds divided into 3 cycles of 30 seconds. Residual NaOCl was inactivated by sodium thiosulfate and gently irrigated with sterile saline. The samples were then serially dilluted and the colony forming units’ (CFU) number was counted by using the culture method. The results of the studies were analyzed using statistical tests: Kolmogorov–Smirnov test, Kruskal-Wallis test, Wilcoxon test, Mann Whitney U and double sided t-test with the level of significance set at 0.05. Results: All tested irrigation protocols successfully eliminated root canal filling remnants from curved root canals after conventional root canal retreatment (p < 0.001). LAI/SSP group showed higher reduction rate than SWEEPS/flat-tip group (p=0.032). There were no significant differences between other groups (p > 0.05). In oval root canals filled with bioceramic sealer, SWEEPS showed showed similar retreatment efficacy as UAI and SSP (p < 0.05). Regarding antimicrobial efficacy, the SWEEPS and UAI showed as equally effective in eliminating the E.faecalis biofilm (p < 0.001), in contrast to SNI which did not present sufficient effectiveness (p = 0.553). Conclusion: The results of the doctoral thesis showed that the SWEEPS is highly effective irrigation technique that successfully removed remnants of epoxy-resin and bioceramic sealers from round curved and oval root canals. SWEEPS modality used with a radial laser tip showed slightly better performance than a flat laser tip. LAI/SSP group showed slightly better effectiveness in removing root canal filling material remnants from severely curved root canals than the other tested techniques.