Bacterial endospores are the most resistant living structures, known. Their high degree of resistance to many treatments (including heat and UV) is due to many factors and is governed by the unique spore structure. Spore core dehydration is a primary determinant of heat resistance. A specialised cell wall peptidoglycan layer, termed the cortex maintains dehydration. During germination the spore cortex is hydrolysed to allow outgrowth of the new vegetative cell. The germination-specific lytic enzymes (GSLEs) responsible for cortex hydrolysis have recently begun to be identified. Their position, in the dormant spore, outside the protection of the dehydrated core requires a special mechanism of heat resistance, which is so far unknown. Recent evidence has begun to elucidate the molecular basis not only for resistance but also how it is acquired, maintained and lost during germination. Understanding of resistance mechanisms may allow the design of novel sporicidal treatments.