N1 or N-1 was the secret Soviet rocket intended to send Soviet cosmonauts to the Moon, preferably ahead of the Americans. It is also known in the west as the G-1e or SL-15. It was underfunded, undertested, and started several years after the Saturn V; all four launch attempts failed, and the project was officially cancelled in 1976.

Development of the N1 began in 1959 under the direction of Sergei Korolev at his OKB-1 Design Bureau. The original design proposed a 50 ton payload intended as a launcher for military space stations and a manned Mars flyby using a nuclear engine upper stage. The N1 was the largest of three proposed designs; the N2 was somewhat smaller and intended to compete with the proposed UR-200, and the much smaller N3, which would replace his "workhorse" R-7 rocket. No actual development was carried out at this point, however; the N-series was strictly a "paper project".

In December 1959 a meeting was called with all of the chief designers, who presented their latest designs to the military. Korolev presented the N-series along with a much more modest series of upgrades to the R-7. Vladimir Chelomei, Korolev's rival, presented his "Universal Rocket" series, which used a common lower stage in various clustered configurations to meet a wide variety of payload requirements. Mikhail Yangel, perhaps the most successful of the three but with little political power, presented the small R-26 intended to replace the R-16, the much larger R-36 ICBM, as well as the SK-100, a space launcher based on a huge cluster of R-16's. In the end the military planners selected Chelomei's UR-100 as the new "light" ICBM, and Yangel's R-36 for the "heavy" role. They saw no need for any of the larger dedicated launchers, but also gave Korolev funding to develop the Molniya (8K78) adaptation of the R-7.

Things changed in 1961. In March, during a meeting at Baikonur, designers discussed the N1 design, along with a competing design, the R-20. In June, Korolev was given a small amount of funding for N1 development between 1961 and 1963. In May 1961 a government report, On Reconsideration of the Plans for Space Vehicles in the Direction of Defense Purposes, set the first test launch of the N1 rocket for 1965.

When the US announced the goal of landing a man on the moon in May 1961, Korolev proposed a lunar mission based on a new spacecraft, eventually known as Soyuz, that was designed for on-orbit rendezvous. Several launches would be used to build up a complete moon package, one for the Soyuz, another for the lunar lander, and additional launches with cislunar engines and fuel. Although this mission profile, "Earth-Orbital Rendezvous", requires the least amount of launch weight of its boosters, the lunar boosters and fuel were simply too large for any existing Soviet launcher. Korolev thus proposed development of the N1 with a 50-ton payload – much smaller than the N1 design that would eventually be delivered.

To power the new design, Valentin Glushko, who then held a near-monopoly on rocket engine design in the Soviet Union, proposed a new engine, the RD-270, running on unsymmetrical dimethylhydrazine (UDMH) and nitrogen tetroxide (N₂O₄) for Korolev's design. This formula is hypergolic (i.e. its components ignite on contact, reducing the complexity of the combustion system), and was widely used in Glushko's existing engine designs used on various ICBMs. However, the propellant pair UDMH/N₂O₄ has specific impulse lower than kerosene/liquid oxygen, and Korolev felt that any realistic high-performance design would need to use higher-performance fuels. Even more importantly, Korolev felt that the toxic nature of the fuels and their exhaust presented a real safety risk.

The difference of opinions led to a falling out between Korolev and Glushko. In 1962 a committee that was appointed to break the logjam agreed with Korolev. Since Glushko refused to work on such a design, Korolev eventually "gave up" and decided to enlist the help of the Nikolai Kuznetsov's OKB-276, a jet engine designer. Kuznetsov, who had limited experience in rocket design, responded with a fairly small engine known as the NK-15, which would be delivered in several versions tuned to different altitudes. To achieve the required amount of thrust, it was proposed that a large number of NK-15's would be used in a clustered configuration around the outer rim of the lower-stage booster. The "inside" of the ring of engines would be open, with air piped into the hole via inlets near the top of the booster stage. The air would be mixed with the exhaust in order to provide thrust augmentation, as well as additional combustion with the deliberately fuel-rich exhaust. The ring-like arrangement of so many rocket engine nozzles on the N1's first stage could have been an attempt at creating a crude version of a toroidal aerospike engine system; more conventional aerospike engines were also studied.

Meanwhile, Chelomei's OKB-52 proposed an alternate mission, LK-1, with much lower risk. Instead of a manned landing, Chelomei proposed a series of circumlunar missions which he felt would be able to beat the US. He also proposed a new booster for the mission, clustering three of his existing UR-200 designs (known as the SS-11 in the west) to produce a single larger booster, the UR-500. However, these plans were dropped when Glushko offered Chelomei the RD-270, which allowed the construction of a much simpler "monoblock" design, also known as the UR-500.

The Soviet military, specifically the Strategic Missile Forces, was reluctant to support what was essentially a politically-motivated project with little military utility. However, both Korolev and Chelomei pushed for a lunar mission. For some time, between 1961 and 1964, Chelomei's less aggressive proposal was accepted, and development of his UR-500 and the L1 spacecraft were given a high priority. When Gemini missions put the US in the lead in space development, Korolev pressured Nikita Khrushchev into making a lunar landing before the US. Since there were a number of unknowns in the Earth-Orbit-Rendezvous profile that could not be tested in time, a direct-launch profile similar to Apollo was selected. This required much larger boosters.

Korolev proposed a larger N1, combined with a new lunar package known as the L3. The L3 combined the lunar engines, an adapted Soyuz spacecraft (the LOK) and the new LK lunar lander in a single package. Chelomei responded with a clustered UR-500-derived vehicle, topped with the L1 spacecraft already under development, and a lander of their own design. Korolev's proposal was selected as the winner in August 1964, while Chelomei was told to continue with his circumlunar UR-500/L1 work.

When Khrushchev was overthrown later in 1964, infighting between the two teams started anew. In October 1965 the Soviet government ordered a compromise; the circumlunar mission would be launched on Chelomei's UR-500 using Korolev's Soyuz spacecraft in place of their own Zond design, aiming for a launch in 1967, the 50th anniversary of the Bolshevik Revolution. Korolev, meanwhile, would continue with his original N1-L3 proposal. Korolev had clearly won the argument, but work on the L1 continued anyway, as the Zond.

Korolev died in 1966 due to complications after minor surgery, and the work was taken over by his deputy, Vasily Mishin. Mishin did not have Korolev's political astuteness or power, a problem that helped lead to the eventual downfall of the N1, and of the lunar mission as a whole.