Pressure ulcers are one of the most potentially devastating complications in individuals confined to a bed or a wheelchair for an extended period. Severe pressure ulcers may form in deep tissues overlying bony prominences, and only at a later stage become visible, or may even induce a full-thickness breakdown of the soft tissues at the affected site. A new type of pressure ulcer known as ‘deep tissue injury’ has therefore been defined internationally. To understand the aetiology of deep tissue injury, health professionals should be able to predict whether or not a certain state of internal mechanical loads in deep tissues, such as tissue deformations and forces per unit area of tissue, would lead to localised irreversible cell damage. Part one of this article explains the concepts of injury thresholds as related to deep tissue injury. Some serious flaws in the classical, commonly used Reswick and Rogers pressure-time curve are analysed, and an alternative contemporary tissue injury threshold, the sigmoid threshold, is suggested. Part two of this article describes recent and ongoing work aimed at defining injury thresholds that are specific for deep tissue injury, standardised and therefore suitable for use with different patients. Clinical implications of current injury thresholds are also discussed, in relation to obese patients and patients with muscle atrophy.