Critically ill patients, particularly those under mechanical ventilation, require analgo-sedation to control noxious stimuli and enhance comfort. Despite their harmful side effects, such as respiratory depression, physical dependence and difficult arousal, opioids are effective in providing a good level of analgesia and comfort. Traditional opioids (morphine and fentanyl) have been shown effective in providing analgesia; however, the respiratory adverse effects and their pharmacokinetics, with an high risk of accumulation, limits their use, especially for a long-term sedation. In the last decade, new synthetic opioids with limited side effects and favourable pharmacokinetics profile, such as Sufentanil and Remifentanil, have been investigated to evaluate their efficacy in mitigating pain and enhancing comfort in critically ill patients.