Flight is a capability many mechanoids and no few Transformers have, whether through rocket boosters, propellers, or even flapping wings. It has proven quite useful strategically in many continuities.
The most common means by which Transformers fly is some variation on jet-propulsion engines, rocket engines, or similar. Transformers such as the Seekers have such thrusters built into their legs, while other individuals such as Sideswipe and Sunstreaker possess removable jet-packs which strap to their backs.
Transformers with jet-powered flight
Sometimes, by virtue of a Transformer's alternate mode, they can fly by means more primitive than they might be accustomed to on Cybertron. Using airplane propellers, helicoptor rotors, or other similar technologies they can fly at speeds comparable to native aircraft on the worlds they are residing upon.
Transformers with propeller-powered flight
Many transformers have a physical pair of beast-mode wings, whether natural-looking or mechanical. In these cases, the transformer's flight derives from flapping these wings or gliding much like an animal would.
Transformers with biological flight
Some Transformers just float, often by no apparent means. It isn't always that well explained in the fiction, though certainly there exist transformers with anti-gravity generators and other technologies which allow them to maneuver effortlessly even within a gravitational field.
Transformers with levitation
Sometimes the Autobots have them....sometimes they don't. Most often they display them early in Season 1 of the US cartoon, particularly the pilot. It is uncertain if they require external equipment to fly; several times the Autobots used strap-on jet packs or rocket packs, but sometimes they don't.
Robot-mode flight was prevalent in the post-movie episodes, in which the focus was shifted to space (perhaps, taking a hint from the Gundam franchise.) As although some characters could not fly under the influence of a planets gravity, 'everyone' seemed to posses the power of flight in deep space.
Generation 1 cartoon
Robot-mode flight powers were a technological innovation that allowed the Decepticons, who had just mastered the art of Transformation, to start a new war and battle the Autobots for supremacy of Cybertron. Their initial attacks were not a matter of public knowledge, and some Autobots, such as Orion Pax and Dion, admired them for their innovations. This admiration was ended when Megatron blasted Orion, Dion, and Ariel. It was to Megatron's detriment, however, for Alpha Trion rebuilt Orion and Ariel into Optimus Prime and Elita One.
Eventually, the Autobots also mastered robot-mode flight powers, and they used them to reach an oil rig which the Decepticons were plundering. Sideswipe's rocket pack to catch up to the space-bound Decepticon ship shortly afterwards, implying a limit on the speed or range of his flight capabilities. Sideswipe and Sunstreaker also used robot-mode flight powers in their use of jet judo against seekers attacking the Autobot base.Ironhide and Bluestreak pursued the fleeing Decepticons into the air, though Skywarp had little difficulty shooting down Ironhide. Optimus required the use of
The Decepticon's own flight capabilities have known to fail them at times too. In one instance, under a barrage of firepower, Optimus and the Autobots drove the Decepticons back until they fell down a canyon into lava, instead of simply flying there.
The Autobots' ability to fly in robot mode effectively vanished after that. To reach Dinobot Island, Optimus Prime and a crew of Autobots used strap-on jetpacks. Even Autobots with flying alternate modes did not seem able to fly in robot mode, though Cosmos seemed able to maneuver in space while in robot mode.
The newly-created Dinobots could fly in robot mode, an abiliy they consistently demonstrated throughout their history. Their creator Wheeljack was also shown flying in robot mode apparently unassisted, an ability his tech spec claims for him.
In the years following the defeat of Unicron, a number of Autobots were seen to fly through space without any visible aid or means of propulsion. Notable among them was Perceptor, who had never before demonstrated such an ability. Before one such instance, Sky Lynx advised the Autobots in question to "don your rocket packs, gentlemen!" The Technobots all flew in robot mode from Earth to Saturn's moon Titan.
Despite the Autobots' rare display of this ability, some claim they do not have it at all. Galvatron once said outright that "Autobots cannot fly," himself flying easily over a booby trap, while his apparently earthbound Autobot companions remained helpless.
Beast Wars cartoon
Early in this series, the Predacons had a tactical advantage due to the imbalance of flight-capable individuals in their faction. Initially, only Optimus Primal could provide air support for the Maximals, and then only in robot-mode which was risky in the energon-rich enviroment. However as more Protoforms were formatted and pressed into service, particularly Airazor and Silverbolt, the balance of forces was more even.
Sufficient volumes of energon itself, of a certain instability, are said to have the ability to cause large objects to float.
Transmutate had an apparently unique flight mechanism: it extended a crystalline object from within its body, and when the crystal was exposed to air it gave off an intense energy field and then caused its user to levitate. Since Transmutate was a malformed, incomplete design, this might have been a display of widespread Transformer flight technology that otherwise would always be internal and unseen.
Back before the war, information on scientific breakthroughs in levitation/"repulsors" that could lead to robot-mode flight abillities catches the greedy optics of Senator Ratbat who manipulates the stock to tack control. Given that he offers a variety of war-tech to the fledgling Decepticons under Megatron this is may be intended as reference to or origin for their robot-form flight abilities; however, the Decepticons have demonstrated no such abilities in the IDW continuity. This may be an artifact of this storyline's planning and even possible scripting during the latter days before the Dreamwave bankruptcy.