The Sanctuary of an Orthodox church is separated from the rest of the nave by the iconostasis. The iconostasis evolved from a chancel rail (called a templon in Greek), a feature still found in many Western churches. A vestige of the chancel rail can be seen in the tops of the base panels under the smaller icons. Over time, screening was extended above the templon. Surviving examples can be found in some pre-10th century Greek churches, but the best preserved example of a pre-iconostasis altar screen can be seen in the Basilica of St. Mark in Venice, Italy. The iconostasis had evolved into its present form by the fourteenth century.

An iconostasis can be as simple as a row of icons attached to a chancel rail (sometimes found in modern Orthodox churches), or it can also be an elaborate, complex dividing wall with many rows of icons stretching to the ceiling. It can be temporarily improvised in a rented room with two icons (the Theotokos and Christ Child on the left, Christ Pantocrator on the right, with a door-sized space between them) on a pair of music stands. In some Russian log churches, the Sanctuary may be a separate room adjoining the nave. In this case, the east wall of the nave becomes the iconostasis. It can even be a portable screen that can be set up in a few minutes. Regardless of the physical form it takes, the iconostasis in all cases serves the same functions: a) reminding believers of the veil in the Jewish Temple that separated the Holy of Holies from the congregation, b) representing the mystical boundary between spiritual and temporal worlds, and c) as with all icons, each icon serves as a "window into heaven."

Only the clergy and altar boys are permitted behind the iconostasis. Anyone with a valid liturgical reason for entering may enter or exit through the Deacon's Doors (numbers 1 and 3), but only a priest or bishop may enter or exit the Sanctuary through the Royal Doors (number 2). With the exception of Bright Week (the week immediately following Easter), the doors are kept closed when there is no service going on. The doors are left open during Bright Week as a reminder that the veil in the Temple was torn open on Good Friday. Visitors are allowed to view the inside of the Sanctuary, but may not enter.

The icons shown above are described on the "West Wall" page.