Although Turkey is an informal country, it is often said that Istanbul isn't Turkey—it's Europe. Expect to see the full spectrum in Istanbul when it comes to style and coverage. You may walk down the street next to a girl in a miniskirt, followed by a woman wearing a head scarf or completely covered from head to toe. Istanbul is a cosmopolitan city, so if you plan on a night out on the town, come prepared to dress accordingly. For men, nice jeans coupled with a clean button-down shirt and decent shoes will usually get you in the door; a jacket and tie are only appropriate for top restaurants in Istanbul, Ankara, and İzmir. Women should feel comfortable wearing fashionable styles but, as in any place, consider what kind of attention you want to attract.
Outside major cities, women would do best to avoid overly revealing outfits and short skirts. The general rule is: the smaller the town, the more casual and, at the same time, conservative the dress.
On the beaches along the Mediterranean, topless sunbathing among foreigners is increasingly common, though not always looked kindly upon by locals. Shorts are acceptable for hiking through ruins, but not for touring mosques. The importance of a sturdy, comfortable pair of shoes cannot be overemphasized. Whether you are in Istanbul, where "everything is uphill," or you're hiking the ruins at Ephesus, you'll be glad you sacrificed style for comfort.
Light cottons are best for summer, particularly along the coast. If you're planning excursions into the interior or north of the country, you'll need sweaters in spring or fall and all-out cold-weather gear in winter. An umbrella is advisable on the Black Sea coast, but as anywhere else in Turkey, as soon as rain begins to fall, people will appear almost magically on the streets to sell cheap umbrellas; so if you don't want to bring one with you, it's almost always possible to find one.
Sunscreen and sunglasses will come in handy. It's a good idea to carry some toilet paper and hand sanitizer with you at all times, especially outside the bigger cities and resort areas. You'll need mosquito repellent from March through October, a flashlight for exploring caves in Cappadocia, and soap if you're staying in inexpensive and moderately priced hotels.