Heaping of used poultry litter reliably induces significant elevations in temperature due to the actions of variety of aerobic bacteria and fungi. Temperatures typically rise to over 50°C in 1- 4 days and may peak at more than 60°C. These temperatures are sufficient to reduce or eliminate a range of pathogens giving rise to the term 'pasteurisation'. A common target for litter pasteurisation is heating to 55°C for a minimum of 3 days. Factors influencing heating potential include carbon to nitrogen ratio, moisture content, oxygen availability and to a lesser extent pH. This paper summarises the key underlying principles and factors influencing litter pasteurisation while a companion paper deals with practices to optimise it.