The commercialization of CAD/CAM resin-composite blocks (RCBs) has rapidly evolved while independent assessment of the mechanical properties has been scarce. Objective: To determine and compare the fracture toughness (KIC), flexural strength (σf) and modulus (Ef) of four commercially available RCBs and one lithium disilicate glass-ceramic CAD/CAM block, tested under dry and aged conditions. Methods: Three dispersed-fillers (DF) RCBs, Cerasmart (CER), KZR-CAD-HR2 (KZR), and Camouflage Now (CAM), one polymer-infiltrated ceramic network (PICN) RCB, Vita Enamic (VE), along with Obsidian (OBS), a glass-ceramic block, were characterized. Blocks were cut into bars (10:1 span-to-thickness ratio) and 6x6x6x14mm prisms (n=25/group); half of the resin-composite (RC) specimens were aged in 37°C distilled water for 30 d before testing. σf and Ef were determined using a three-point bending test, whereas KIC was determined through the notchless triangular prism specimen KIC test. Fractured KIC surfaces were characterized with a scanning electron microscope. Results were analyzed using Weibull statistics and two-way ANOVA, followed by Scheffé multiple means comparisons. Results: With regards to σf, OBS>CER=KZR>CAM>VE and σf of RCs was lowered by ageing. VE was found to have the highest Ef among RCs (33.0 GPa), but was significantly lower than OBS (76.5 GPa); Ef was not affected by ageing. With regards to KIC, KZR stood out among RCs with a dry value of 1.4 MPa·m¹/², which was significantly affected by ageing, while KIC of the other DF RCs was not. OBS had the highest KIC at 1.5 MPa·m¹/². Conclusions: Compared to PICN, DF RCBs exhibited significantly higher σf and lower Ef values, while for KIC, only KZR was found to be superior. The tested glass-ceramic had higher σf, Ef and KIC when compared to RCBs, with the exception of dry tested KZR that did not differ significantly for KIC. Ageing had a deleterious impact on σf of all RCBs while its effect was not significant for Ef. With regards to KIC, ageing significantly lowered the mean value for KZR while it increased for PICN. In light of the mechanical testing results, PICN seemed more promising than DF materials but did not surpass blocks made of lithium disilicate.