Rounds are the progression system throughout Zombies mode, increasing after the player has cleared a certain number of Zombies at a time. This also marks the increase in health in most enemies of the game, with a few exceptions.
After every round has been completed, a round transition song plays, the song dependent on the round played on. On normal rounds, it plays the end of "Hell's Gate," then the iconic three-note sound after. After completing Boss rounds, however, it plays a part of "Beauty of Annihilation," and then the three-note sound. The tally marks/numbers would flash white until the next round starts, except in Black Ops 3, where it simply burns in a new tally/number.
"Hell's Gate" variants are found throughout the maps, depending on what map is played.
- Nacht der Untoten, Verrückt (World at War, ZOMBIES, & Black Ops), Shi No Numa (World at War, ZOMBIES, & Black Ops), Der Riese, Kino der Toten (Black Ops & Black Ops Zombies), "Five", Ascension (Black Ops & Black Ops Zombies), Call of the Dead, Shangri-La (Black Ops), Moon (Black Ops), Green Run, Nevada, U.S.A., Great Leap Forward, and Revelations (Nacht der Untoten section) all have the classic song.
- Alcatraz Island and Revelations (Mob of the Dead section) both have a very subtle piano and orchestra song.
- Resolution 1295 and Revelations (Buried section) both have a "Wild West" themed song.
- Origins and Revelations (Origins section) both have an ancient song.
- The Giant is the choir from Mob of the Dead minus the instruments.
- Shadows of Evil has a jazz themed song.
- Der Eisendrache and Revelations (Der Eisendrache section) both have a female choir song, similar to Mob of the Dead.
- Zetsubou No Shima, Shi No Numa (Black Ops III), and Shangri-La (Black Ops III) has a mystical and oriental song.
- Gorod Krovi has a symphony themed song.
- Revelations (main section) has a paradise choir and lullaby theme.
- Revelations (Verrückt section) and Verrückt (Black Ops III) have a bell sounded song.
- Revelations (Kino der Toten section) and Kino der Toten (Black Ops III) is a mix of Buried and The Giant.
- Ascension (Black Ops III) has a science-fiction theme.
- Moon (Black Ops III) has a different type of science-fiction theme than Ascension's.
"Beauty of Annihilation" is played after completing a boss rounds, with the following bosses:
- Hellhounds (every 4-7 rounds)
- Pentagon Thief (every 4-7 rounds after the power has been turned on)
- Space Monkeys (every 4-7 rounds after the power has been turned on)
- Jumping Jacks (every 4-7 rounds)
The round counter is found on the bottom left corner on consoles and PC, and on iOS and Android devices, it's on the top-left corner. In World at War, Black Ops, and Black Ops II, the tally marks are used from round 1 to 10 (I, II, III, IIII,
IIII, IIII I, IIII II, IIII III, IIII IIII, IIII IIII). In ZOMBIES and Black Ops Zombies, the tally marks are used from round 1 to 9 (I, II, III, IIII, IIII, IIII I, IIII II, IIII III, IIII IIII). In Black Ops III, the tally marks are used from round 1 to 5 (I, II, III, IIII, IIII). After all the tally marks are passed, regular digit numbers are used to count the rounds.
In Dead Ops Arcade and Dead Ops Arcade II: Cyber's Avengening, the at the end of each round, the in-game music continues to play and the player must move to a different area, provided by the arrows.
In Moon in Area 51, the round remains what it is on Moon, however, a separate round system is present, and is marked by an air raid alarm. This would sound at random moments, usually every 30 seconds, and the Zombies' health would gradually increase. Reentering the place would continue the round system and leaving would give the player a timed result. Dying here would also give a timed result, however, it replaces the round number.
In Turned mode, the round system is replaced by a timer (6:00 and counting down). At the last ten seconds, the counter then turns to tallies counting down. Once the timer goes to 0, the game ends.
In Buried, players are able to reach round ∞ during the Richtofen side of the "Mined Games" Easter Egg. Zombies would be invincible, and are not able to die unless killed with an infinite-damage weapon, such as the Ray Gun Mark 2.