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Since resin composites were first introduced to dentistry, their formulations have been developed significantly. Recently, a new class of ultra-rapid photo-polymerized bulkfill 'URPBF' resin-composites has been introduced. Due to the incorporation of Addition-fragmentation chain transfer (AFCT) agent, these materials are claimed to photopolymerize within only 3 s under high irradiance from a PowerCureÔ lightcuring unit (LCU). Therefore, the focus of this thesis was to study early-stage properties of this new composite system, during and following photopolymerization, including methods-development to measure mechanical-property evolution. Using FTIR spectroscopy, the degrees of conversion, conversion kinetics and their development post-irradiation for these URPBF materials were measured at both 1 mm and 4 mm depths, sub-surface. Following 3 s high-irradiance, PFill and PFlow materials were found to be broadly comparable to their predecessors: ECeram and EFlow (both irradiated for 20 s), over 24 h post-irradiation, at both depths. Polymerization shrinkage and shrinkage-stress development were studied via bonded disk and Bioman II methods to determine effects of high irradiance photopolymerization, using 2 LCUs. URPBF material, PFill, had reduced polymerization shrinkage under high-irradiance (1.6 % and 1.7 % for PowerCureÔ and VALOÔ LCUs, respectively) compared to 10 s (1.8 %). These results were also lower than for its predecessors, ECeram (2.7 %). PFill exhibited slightly higher shrinkage-stress (2.2 MPa and 2.3 MPa) under highirradiance from PowerCureÔ -3s and VALOÔ, respectively) compared to 10 s curing (1.8 MPa), but the stress was still lower than the comparator, ECeram (2.6 MPa). Similar behavior was seen with PFlow, except for the 3s VALOÔ group which produced significantly higher shrinkage stress (3.6 MPa). The exothermic reaction during intra-dental photo-irradiation and the effect of high irradiance on temperature fields in situ were also investigated by thermography. This produced 2D temperature maps and temperature/time plots. The maximal temperature rise (?T) was seen within the material bulk, at 2 mm depth. This was significantly higher in PFill and PFlow polymerized via PowerCureÔ -3s (21.8 and 27.1 °C), compared to the same LCU in standard 10 s mode (17.8 and 22.9 °C). However, PowerCureÔ -3s generally produced comparable temperature rise to Elipar S10Ô- 10 s. Nonetheless, only 1 mm of remaining dental thickness was sufficient to ensure a minimal temperature rise at 1 mm within dentine. This suggests freedom from significant thermal damage from rapid photopolymerization and thus the clinical safety of this treatment. A method to study early surface viscoelastic integrity was proposed, using a flat-ended macroscopic indentor axially aligned over each specimen immediately (< 2min) postirradiation. This applied 14 MPa compressive stress for a period of 2 h. Then, indentation-recovery measurements were made on unloaded specimens. This indentation-creep method characterizing URPBF surface integrity immediately postirradiation. Flowable materials showed deeper indentation compared to non-flowable bulkfills. PFill and PFlow polymerized in 3 s resulted in immediate indentations comparable to their counterparts: ECeram and EFlow. With delayed indentation at 24 h, no differences were detected between materials in maximum indentations regardless of the material type or curing protocol. This is attributed to further network development. The ability of this indentation-creep method to characterize several representative resin-composites, including RBCs with varied clinical applications, was then assessed. The method was successfully applied to eight representative RBCs, including three bulkfills, three conventional non-flowables, and conventional flowables. Network developments by 24 h significantly improved the indentation resistance of all materials. Viscoelastic properties evaluated by this method confirmed that highly filled RBCs were more indentation resistant.