This in vitro study determined the fracture toughness of four classes of resin using wet and dry test environments, following 48 hours of wet storage. Two of the resins were light initiated, while the other two were chemically initiated. No significant difference in fracture toughness was detected for the wet and dry test environments using miniature compact tension specimens. The light initiated, urethane dimethacrylate resin demonstrated a significantly higher fracture toughness than the poly(methyl methacrlate) resin.