What currency should you bring to Istanbul: Turkish lira, US dollars, or euros, and do tours need to be paid in euros?

You generally do not need to buy euros specifically for tours in Istanbul. If a tour is priced in foreign currency, operators often convert it at the current rate and accept payment in US dollars or Turkish lira.

If you bring US dollars, they will be exchanged into Turkish lira at the money changer’s or bank’s current rate on that day, so there is no fixed advance rate to rely on.

In practice, it is more convenient to keep most of your travel money in Turkish lira for day-to-day expenses in Istanbul, and carry some US dollars as a backup for exchange or occasional foreign-currency payments.