If you have dandruff, you are not alone. Approximately 50 million Americans and 50% of the adult population worldwide have the common scalp condition in which small pieces of dry scalp skin fall off in flakes. Many people assume dandruff is a sign of poor hygiene or not shampooing enough—it’s not! Scientists don’t know what causes it, but several factors make you more prone to having dandruff, including the presence of fungal colonization, the oiliness of your skin, and your personal tendency towards dandruff. Fortunately, dandruff often can be treated effectively.