Estimates of the frequencies of rare disorders vary from country to country; the global average defined prevalence is 40 per 100 000 (0.04%). Some occur in only one or a few patients.However, collectively rare disorders are fairly common, affecting 6–8% of the US population, or about 30 million people, and a similar number in the European Union. Most of them affect children and most are genetically determined. Diagnosis can be difficult, partly because of variable presentations and partly because few clinicians have experience of individual rare disorders, although they may be assisted by searching databases.Relatively few rare disorders have specific pharmacological treatments (so‐called orphan drugs), partly because of difficulties in designing trials large enough to determine benefits and harms alike. Incentives have been introduced to encourage the development of orphan drugs, including tax credits and research aids, simplification of marketing authorization procedures and exemption from fees, and extended market exclusivity. Consequently, the number of applications for orphan drugs has grown, as have the costs of using them, so much so that treatments may not be cost‐effective. It has therefore been suggested that not‐for‐profit organizations that are socially motivated to reduce those costs should be tasked with producing them.A growing role for patient organizations, improved clinical and translational infrastructures, and developments in genetics have also contributed to successful drug development.The translational discipline of clinical pharmacology is an essential component in drug development, including orphan drugs. Clinical pharmacologists, skilled in basic pharmacology and its links to clinical medicine, can be involved at all stages. They can contribute to the delineation of genetic factors that determine clinical outcomes of pharmacological interventions, develop biomarkers, design and perform clinical trials, assist regulatory decision making, and conduct postmarketing surveillance and pharmacoepidemiological and pharmacoeconomic assessments.