Stealth

Stealth is a mechanic which is first introduced in Update 7. It offers a better and easier time doing full stealth runs. This is only the first update of the like, so expect tweaks in the near future.

How to prompt an assassination
As long as the crew has not been alerted to your presence, you are eligible for performing stealth attacks. In order to do so, approach an enemy from behind without alerting anyone to your presence, and press your melee key when the prompt appears. Depending on what melee weapon you are using, different animations will occur. Successfully pulling off a stealth attack will deal damage with a large multiplier (Damage is affected by the weapon's Charge Damage). So far, the animation varies by type, where daggers, swords, heavies, staff, Glaive, gauntlets and dual daggers all have their unique animations.

Enemy eligibility for stealth attack
These enemies cannot be stealth killed:

Grineer: Corpus: Infested:
 * Powerfist
 * Roller
 * Shield Lancer
 * MOAs
 * Ospreys
 * Cameras
 * Turrets
 * Ancients

Detection

 * Noise caused by breaking objects (containers, fans, windows), your unsilenced weapons or manipulating based abilities (Decoy, Pull etc.) will alert enemies. Opening doors, all movements of the player will not attract attention. Certain weapons make less noise (Paris, Kunai, Despair; certain weapons with very good range like Latron, Snipetron and Vulkar, at the right distance), and thus are less likely to attract attention.
 * Their range of view before in alert is <180 degrees, so crouching still in a corner is a legitimate way to expose the back of a Grineer that just walked through a door. They are also bad at detecting objects at a distance and anything somewhat above them. Hiding behind terrain will conceal your presence.
 * When enemies are in alert, they will become highly sensitive to your presence. Their viewing cone will be wider, and will be able to spot you or your sentinel even from a very far distance, unless you are cloaked.
 * Performing an unsuccessful stealth kill will alert the victim (happens a lot when in a high level area, or on heavy Grineers with shields).
 * Having a Sentinel equipped may be detrimental to your stealth ability, as it does not hide behind low cover.
 * In order to put the whole level in alert, an enemy needs to activate a control panel. Killing everyone before a control panel is touched can still allow you to remain undetected. However, gunfire will put enemies in other map tiles in alert temporarily. These enemies keep watch behind cover, or patrol with their guns raised and are more sensitive to your presence. They will "cool down" and can be stealth killed after a short period of time.
 * Cameras make loud alarms when they see you, but again, if they are destroyed and nothing is in range to take advantage of their alert, you're safe. Rooms with two cameras are difficult to get past undetected without destroying them.

Silenced Weapons

 * Bows (Paris, Dread) are completely silent, able to multi-kill and pin corpses out of sight.
 * All melee weapons, including the Glaive.
 * Throwing weapons: Despair, Kunai. Contrary to the bows, although advertised as silent, attacking enemies with throwing knives will still alert others nearby, regardless whether it is a one-shot kill or not. This may be a bug.
 * Some offensive and cloaking based warframe abilities (Shuriken, Teleport etc.): silent if done correctly, and does not disrupt Shade's cloak. However there is a bug where enemies in the next map tile may be alerted temporarily.

Tips

 * Be patient.
 * Crouch for better sneak runs. You'll have a smaller model so you can hide your silhouette better. The speed at which you move is also more convenient for timing your melee than running, and you will be less likely to bump into the enemy before the prompt appears. Also, it seems that enemies are less likely to notice you while you are crouching.
 * If enemies see a corpse that has not disappeared yet, they will be alerted for a short period of time.
 * If you have a Shade with Ghost ability, consider sprinting toward an enemy to activate it quickly.
 * Rolling is a great way to move quickly while crouching.
 * Beware of breakable reinforced glass, unless it is part of the player's tactic to cause a controlled lock down to eliminate all enemies inside a chamber. This action will not cause a high alert.
 * If the prompt does not appear, consider doing a charge attack.

Current grievances in the mechanics
Since the concept of stealth is still a new one, there are some flaws that have not been addressed.
 * The practicality of attempting assassination on heavy and shielded units.
 * If you aren't at the same altitude as the target, the prompt will not appear easily.
 * Prompt will appear and disappear unexpectedly when the enemy are in alert.
 * Not all enemies can be stealth killed. It is hard to know whether an enemy type can be stealth killed until the player approaches behind the target.
 * After each stealth attack, you finish standing up, which might reveal your position. You will need to release and re-press the crouch button to return to crouch position. If you are using a toggle crouch key, simply press it during the animation to return to crouching when the attack completes.
 * There appears to be no inherent benefits of stealth gameplay aside from the prompt for an assassination and lower enemy spawn rate, which is counter productive given that weapons level via kill exp.