An endospore is any spore that is produced within an organism (usually a bacterium). This is in contrast to exospores , which are rather produced by growth or budding. The primary function of most endospores is to ensure the survival of a colony through periods of environmental stress. Endospores are therefore resistant to desiccation, temperature, starvation, ultraviolet and gamma radiation, and chemical disinfectants.

Spore position in the mother cell or sporangium differs among species and is useful in identification. The spore is often surrounded by a thin covering called exosporium. A spore coat lies under the exosporium. The spore coat is impermeable to many toxic molecules. The coat may also contain enzymes that are involved in germination. The cortex lies beneath the spore coat. The cortex consists of peptidoglycan. The spore cell wall (core wall) is inside the cortex and surrounds the protoplast or core. The core has normal cell structures but is metabolically inactive. Up to 15% of the spore's dry weight may consist of dipicolinic acid complexed with calcium ions. Dipicolinic acid could be responsible for the heat resistance of the spore. Calcium may aid in resistance to heat as well as oxidizing agents. The combination of calcium ions and the dipicolinic acid may stabilize spore nucleic acids.

As a simplified model for cellular differentiation, the molecular details of endospore formation have been extensively studied, especially in the model organism Bacillus subtilis. These studies have contributed much to our understanding of the regulation of gene expression, transcription factors, and the sigma factor subunits of RNA polymerase.

The endospore-forming bacteria belong to the Firmicutes. Sample genera include:

• Bacillus
• Clostridium
• Desulfotomaculum
• Sporolactobacillus
• Sporosarcina
• Thermoactinomyces