The most common cause of an itchy, flaky scalp is dandruff or its more inflamed cousin, seborrhoeic dermatitis, both linked to a yeast that lives on the skin and to oil production. This produces white or yellowish flakes and sometimes redness, and it tends to come and go, often worse in cold weather or with stress.
Other causes include scalp psoriasis (thicker, silvery, well-defined scale), contact reactions to hair products, and fungal infections such as ringworm of the scalp, which can cause patchy hair loss and needs specific treatment. Over-washing or harsh products can also irritate the scalp.
Medicated anti-dandruff shampoos available over the counter help many people; a pharmacist can suggest a suitable option. See a doctor if there is significant hair loss, painful or pus-filled spots, a rapidly spreading rash, or if the problem does not improve with regular treatment, as some conditions need prescription therapy. This is general information, not a diagnosis.