Built in the early 17th century in a style that emulates its neighbor, the Hagia Sophia, the Blue Mosque manages to outdo its predecessor with a total of six minarets. Another huge difference between the Hagia Sophia and the Blue Mosque is that the former has been turned into a museum that allows tourists to enter at all times during the day, while the active Blue Mosque is not only closed off from non-worshippers five times during the day, but all entrants must remove their shoes and all women are required to don head coverings. Also, be sure to pay the utmost respect to the everyday people engaged in prayer, for this is far less a tourist attraction (despite its prominence among them) than a sacred place of worship.
Entrance is free and best attempted at midmorning or at least a half an hour after the Ezan is sung; otherwise you might be barred from entering until services are done. Persevere if you must; the Blue Mosque is an essential stop on your Istanbul cultural itinerary.