A dietitian or dietician (dietitian is the official spelling used within the dietetics profession) is an expert in food and nutrition. Dieticians: Dietitians help promote good health through proper eating. They supervise the preparation and service of food, develop modified diets, participate in research, and educate individuals and groups on good nutritional habits. In a medical setting, a dietitian may provide specific artificial nutritional needs to patients unable to consume food normally. Dietary modification to address medical issues involving dietary intake is also a major part of dietetics. The goals of the dietary department are to provide medical nutritional intervention, obtain, prepare, and serve flavorsome, attractive, and nutritious food to patients, family members, and health care providers.

In many countries only people who have specified educational credentials can call themselves "dietitians" — the title is legally protected. The term "nutritionist" is also widely used; however, the term nutritionist is not regulated as dietitian is. People may call themselves nutritionists without the educational and professional requirements of registered dietitians.

Different professional terms are used in different countries. Dietitians are a valuable member of the medical multi-disciplinary team providing nutritional knowledge and acting as consultants to other health care professionals.