Strength and fracture properties of single and polycrystalline CaO are presented along with some optical and hydration properties. Single crystals have optical transmission from about 0.2 to 12 μm, wider than MgO in both the infrared and ultraviolet, but develop absorption, particularly at 2.75 μm, from atmospheric attack. They have the same cleavage and slip systems as MgO, but are softer. The yield stress, which is sensitive to impurities, is from 2 to 6 × 103 psi at room temperature for purer crystals and is about doubled at liquid nitrogen temperatures. Room temperature strengths of polycrystalline CaO as high as 30,000 psi appear to be primarily determined by dislocation phenomena and hence are sensitive to impurity content, state, and distribution. Lower temperature behavior appears to be determined more by flaws, whereas high temperature strengths can be severely limited by grain boundary sliding enhanced by small quantities of impurities. Sanding of both single and dense polycrystalline bodies can increase strength, apparently because of surface work hardening, but reduces crystal ductility. Testing in water or after exposure to moisture has at most a limited effect on strengths of dense polycrystalline CaO, whereas single crystals have the same or greater ductility in water as in air. Moisture attack on such bodies is usually quite slow, often taking several months or longer for serious attack, indicating some possibilities for applications without protection. Cracking from moisture attack proceeds by wedging action resulting from the larger volume of the hydrate forming in a pre-existing crack, flaw, or void.