The doses of opioids needed for pain relief vary between individuals. Traditionally, this variation has been explained by variable bioavailability and differences in intensities of pain stimuli. However, studies of cancer pain have shown that pharmacokinetic differences or differences in the extent of cancer disease do not explain all variability in the doses needed for adequate pain relief. Basic science researchers have established the existence of genetic variability associated with genes encoding proteins involved in opioid pharmacology. Such experimental findings have inspired research groups to search after genetic factors that explain clinical variability of opioid efficacy. Given the complexity of morphine pharmacology such variability can occur at several genes. In this workshop we will discuss the results from studies on genetic variation in genes related to morphine pharmacokinetics, the gene encoding mu-opioid receptors and genes encoding multidrug resistance transporters which participate in the transport of opioids trough the blood-brain barrier. We will also present examples how genes encoding proteins not directly involved in opioid pharmacology may influence the patients need for an opioid drug. The aim of this part of the workshop is to give a short introduction to the some of the basics of human genetics. We expect that future research will reveal genetic variability at several genes encoding functions related to opioid pharmacology and at genes related to other symptoms in palliative care patients. Hence, some insight into genetic issues is important in order to understand why patients and drugs often interact in unforeseen ways.