The invasion of a system can be broken into five discrete phases: Discovery, Approach, Attack, Subdual and Consumption.

Discovery of potential targets is under taken primarily by warp-capable drone-ships which disperse among the star systems close to a hivefleet. Over the course of decades these living ships methodically investigate planets and, upon discovery of likely candidates, seed worlds with 'vanguard' organisms such as Lictor and Genestealer infiltrators and various Gaunt species. It is believed that the psychic signature of these creatures draws the hivefleet to them proportionally to their numbers. If the Tyranid nests are fruitful and multiply quickly the hivefleet is soon drawn to the apparently rich feeding grounds, less successful colonies appear to be given a lower priority.

This method is also employed by the use of wrecked space hulks to carry pockets of Genestealers ahead of the hivefleet on the uncertain currents of the Empyrean. If the creatures arrive at an inhabited world they can infect its populace with a genetic corruption and achieve a vanguard of hybridised slaves which will also attract the attention of a hivefleet. The success of this approach can be argued by the fact that Genestealers have been encountered aboard space hulks in every Segmentum of the galaxy. Likewise Genestealer infestations are commonplace enough that the High Lords have issued no less than eighty decrees pertaining to their cleansing by Exterminatus in the last decade.

If all else fails it is believed that the hive-ships perform some form of spectrographic analysis of the radiation from the nearest stars to select a destination. This has been born out by the fact that 72.3% of encounters with hive-ships (as opposed to the smaller drones) have taken place in systems containing class F2IV -K9V stars. This spectral band covers 81% of the worlds inhabited by humanity.

The arrival of the hivefleet is preceded by what the Adepts term 'The Shadow In the Warp', a psychic disturbance which renders Astropathic communication impossible, blots out the Astronomicon and renders warp travel extremely unpredictable. Both the relativistic distance and size of the Hivefleet affect the intensity of the warp-shadow, but in several reported instances entire sectors have disappeared beneath it. As such the shadow in the warp forms an uncertain compass of the Tyranids movements, all too often the target of their attack becomes apparent only after it has been enveloped and rendered unapproachable.

The bio-ships which comprise the fleet may either exit the warp simultaneously at the edge of the system or, in many reported instances, exit in deep space and drift towards their objective. This latter approach renders the Hivefleet virtually invisible to long-range augury and Astrotelepathic detection. There has been speculation that some hivefleets may actually drift through realspace for the entire voyage between systems, but as this would entail a journeytime of decades or centuries of hibernation it must be consigned to the realm of conjecture.

The Tyranid invasion is begun by the hiveships releasing spores into the target's planets atmosphere. The mix typically includes mycetic spores for landing 'vanguard' and 'main force' ground assault swarms, spore 'mines' of a corrosive, poisonous and/or explosive capability, zoomorphic symbiotes and parasites, plus fertilisation, weather modification and necrotic varieties. Flying creatures are also dispatched, primarily swarms of winged Gaunt species tended to by the dropship-sized Harridan brood organisms.

In the initial phases of invasion the flying swarms make concentric attacks in an effort to herd lifeforms into the path of ground swarms. Pre-existing Vanguard organisms will move to join the advance, presumably sharing their information of local DNA, topography and resistance with higher synaptic receptors of the Tyranid hive mind. Battles are frequently reinforced and expanded by the direct dropping of reinforcements in mycetic spores from the hive ships in orbit.

The ground swarms are continuously reinforced with increasingly powerful contingents of Tyranid Warriors, bio-artillery and specialised organisms like the Carnifex assault-beasts and Hive Tyrants. As resistance lines stabilise they are infiltrated or smashed with coordinated wave attacks and mycetic spore drops. Centres of resistance are surrounded, besieged by the gathering of increasingly large and powerful swarms until they are overrun.

Strictly speaking the consumption of the planet under attack is undertaken continuously from the moment the hiveships achieve low orbit and release organisms into the atmosphere. However, the point upon which the hiveships begin to replenish their reserves of bio-mass from the planet is a discrete phase which forms the culmination of the Tyranid assault.

Initially emerging Tyranid feeder organisms spring up in the wake of the ground swarm's advance, mobile ones following hard at its heels while sessile varieties spread and multiply at a ferocious rate, hybridising or absorbing native life. Carrion from the battlefields is quickly consumed and used by several species to pupate and metamorphose into larger lifeforms. Bio-factory organisms harvest and transmute the remaining feeders into nutrient gels which are employed to feed the fighting swarms and droneship-haulers which replenish the hiveships in orbit.

During this period, climate and humidity is subject to modification, as increasing numbers of Tyranids organisms manipulate weather patterns to achieve maximum growth, carbon dioxide and nitrogen are leached away, leading to an increasingly oxygen rich environment. As resistance is overcome more and more of the planet's surface is stripped bare by continuous harvesting until the hivefleet concludes its actions by draining the planets' atmosphere and seas. At this point many hiveships will calve, adding to the numbers fleet's numbers of drone-vessels and immature bio-ships. The attack complete, the hivefleet will then move into deep space again to seek a new target. Tyranid hivefleets typically comprise of a mix of large Hive ships and larger quantities escortsized 'drone ships'. The smaller vessels generally act instinctively and are only unified into a coherent strategy by the psychic emanations of the hiveships. The need for the hiveminds' overall control to be channelled through synaptic links makes Tyranids fleets ponderous but well co-ordinated, a factor which is more than compensated by their overwhelming numbers.