The traditional restorative materials based on amalgam are being replaced by resin composite materials. The wide acceptance of this material is due to several characteristics: it is easy to use, aesthetically pleasing and requires less cavity preparation. Furthermore, it carries fewer environmental risks. Concerns about the use of this material and its performance in the long term are due to issues such as recurring caries and bulk fracture. Clinical data suggest that the most common problems are restoration fracture and secondary caries, therefore these issues deserve investigation. This will require an in-depth exploration of the factors that influence degradation resistance in photo cured fibre reinforced composites. This question is investigated in this thesis, which focuses on the polymeric phase of fibre reinforced composites. This thesis comprises of four expermintal chapters. In chapter 4 and 5, Three resin-composites incorporating fibres, additional to particle reinforcement, were examined: everXâ¢, NovoPro Fillâ¢ and NovoPro Flowâ¢. Four composites were used as controls, with only particle reinforcement: Filtek bulk Fillâ¢, Filtek bulk oneâ¢, Filtek XTEâ¢, and Filtek Flow XTEâ¢. For hardness measurement, specimens were stored dry for 1 h and then in either water or 75% ethanol/water for 1 h, 1 d and 30 d at 37 Â± 1Â°C. VHN decreased for all composites with storage time in both solvents, but more appreciably in 75% ethanol/water. For fracture toughness (KIC) measurements, single-edge-notched specimens were prepared and stored for 1 and 7 d in water at 37Â°C. KIC ranged from 2.14 (everX Posterior) to 0.96 NovoPro Flow) MPa.m0.5. For sorption and solubility measurements: Over a period of 140 d specimens were weighed at predetermined time intervals, then they were dried for a further 42 days at 37Â±1Â°C, to assess the desorption. After 140 days in storage, the water sorption values were found to range between 19.96 and 30.11 Î¼g/mm. XTF exhibited the highest sorption, followed by EVX and NPF, for which both results were similar. In terms of solubility, the range was between -1.49 to 5.28 Î¼g/mm; NPF and XTF were found to have the highest solubility levels, with EVX demonstrating a negative solubility value at -1.49 Î¼g/mm. The hygroscopic expansions at 140 days exhibited results between 1.40 and 2.21%. everX had the highest expansion (2.21%), while NovoPro Fill had the lowest expansion (1.40%). Within chapters 6, and 7 five experimental fibre-reinforced resin composite materials were evaluated. Controlled changes were made to its matrix chemistry in order to determine how the materialâs mechanical and physical properties were affected.The following monomer mass fractions were mixed: 50% bis-GMA plus 50% of different ratios of Bis-EMA+UDMA to produce consistent formulations (Groups B-E) of workable viscosities. As control (Group A), a monomer mixture with mass fractions: 60% Bis-GMA, 30% TEGDMA, 10% PMMA (typical FRC monomers) was also studied. Surface micro-hardness profiles were used as an indirect method to assess the depth of cure (DoC) of experimental fibre reinforced composites. The depth corresponding to 80% of max. VHN, ranged from 3.4 to 4.4 mm. Group B (highest amounts of Bis-EMA) had the greatest DoC (4.4 mm) while group E (lowest Bis-EMA content) had the lowest (3.4 mm). To assess the degree of conversion (DC) moulds with clinically relevant depths (2 mm and 4 mm) were used over the crystal of an (FTIR) spectrometer. DC was measured immediately post-cure. At 2 mm thickness, the lowest DC values were found in group D and E. While, the highest DC values were observed in group A and B, which were significantly higher than other groups. Similar patterns were also observed in the other specimens at 4 mm thicknesses. For FS measurement, materials were cured in 2 x 2x 25 mm by a LED source. Specimens were stored in water for 1 d, 7 d and 30 d at 37 Â± 1Â°C. For KIC measurements, single-edge-notched (SEN) specimens were prepared: 32 x 6 x 3 mm for 3-point bending and stored for 1, 7 d, and 30 d in water at 37Â°C. FS decreased for all composites with storage time in water, but more appreciably in group A after 30 d of storage. While for KIC no significant difference was found for each group stored at different intervals (1d, 7 d, and 30 d). At 30 d storage, the experimental composites showed FS, KIC values not significantly different from each other (P>0.05). The results from this study have shown that Bis-GMA/TEGDMA-PMMA and Bis-GMA/Bis-EMA-UDMA based fibre reinforced composite have varying levels of water sorption and solubility, depending on the type and amount of monomer used. The sorption value of the control group was the highest at 37.8 Î¼g/mmÂ³ and that of experimental composite (group C) was the lowest at 22.6 Î¼g/mmÂ³. This suggests that water storage has a significant effect on the properties of these polymeric matrices.