Objectives: The aim of this study was to clinically compare the bioactive behavior of three indirect pulp capping materials through 18 months. Materials and Methods: Forty five posterior teeth in fifteen patients aged (18-35) years were used in the study. These were randomly divided into three equal groups (n=15) according to the pulp capping material used, Group A (ProRoot MTA), Group B (Biodentine), Group C (ACTIVA BioACTIVE-BASE/LINER). Carious dentin was removed by sterile excavator & smart burs leaving the caries affected dentin. Cavities were restored using Filtek Bulk Fill composite resin. Remaining dentin thickness was measured using preoperative and follow-up periapical digital radiographs. Post-operative sensitivity was evaluated using (USPHS) criteria at baseline, 3, 6, 12 and 18 months. Pulp vitality was assessed using pulse oximeter. Statistical analysis: ANOVA test was used to compare the three groups within each duration and to compare the durations within each group. Pairwise Tukey test was used to compare each two groups regarding the same duration. Chi square test was used for the nominal data. Results: Remaining dentin thickness of all groups gradually increased with no significant difference among the groups (p>0.05), and a highly significant difference among the durations within each group (p<0.001**). Mild post-operative sensitivity that was recorded at baseline in all cases of the three groups, reduced after 3 months and disappeared at 6 months with a significant difference between durations (p<0.05). Conclusions: ProRoot MTA and Biodentine have significantly high performance in indirect pulp capping procedures. Although; ACTIVA BioACTIVE-BASE/LINER has shown promising results.