Methods: a literature review from 1990 to August 2012. Introduction: pain and its recognition can be a particular problem for patients in intensive care units (ICUs). Studies have suggested that around 70% of ICU patients have unrecognised or undertreated pain. Pain has serious physical and psychological effects, and can impair patient recovery and discharge. Pain relief is also an ethical and professional responsibility of doctors and nurses-and we may be failing in this. Causes: pain may be due to medical and nursing procedures, and the ICU environment. Pain can be under-recognised because ICU patients are often impaired in their ability to communicate (e.g. secondary to confusion from acute illness, endotracheal intubation, or reduced conscious level from sedative agents). Tools for pain assessment: in patients able to communicate verbally, the Numerical Rating Scale (NRS) can be used to rate pain severity. In non-verbal, conscious, patients, the Visual Analogue Scale (VAS) can be used as a visual alternative. Both are well-established. For unconscious/sedated patients, the Behavioural Pain Scale (BPS) and Critical Care Pain Observation Tool (CPOT) have been developed and validated. Changes in practice: where possible, sedation practice can be changed to allow better recognition of pain. Constant deep sedation can be interrupted with daily "sedation holds" to allow pain assessment. "Analgo-sedation" may also be used, with drug regimes which prioritise analgesia over sedation. "No-sedation" approaches may also be considered, but further research is required.