"We've always taken these things for granted. They turned a scientific nightmare into a real possibility. We often wondered how we'd traverse the galaxy with only FTL to help us...but the mass relays make such travel instantaneous! I can't even begin to fathom where we'd be without them...but I can't help but wonder. Did the protheans really build them? What evidence do we have to support that? There's a technology behind them that...escapes understanding. If we were to pick them apart, what discoveries would we find inside?"

— Chorban regarding the mass relays.

A mass relay was a large mass transit device, part of a pan-galactic network scattered throughout the Milky Way Galaxy, that permitted near-instantaneous travel across the galaxy within minutes. Hundreds of thousands existed across the Milky Way, strategically placed in clusters and nebulas, functioning off a power-hungry element zero core that powered a FTL engine capable of propelling a ship at nine thousand times the speed of light to reach another part of the galaxy within seconds or, depending on distance, minutes. This greatly sped up the expansion of an alien civilization once discovered, as a process that would normally take tens of thousands of years is reduced to within a human lifetime. However, the mass relay network, as well as the Citadel, were later discovered by most of these civilizations to be creations of the Reapers: a network specifically designed to evolve civilizations along linear technological paths that they can predict and measure, allowing the Reapers to maintain total technological superiority. Dependence on the network for travel also allows for the Reapers to cripple entire civilizations upon their arrival, utilizing the Citadel (which is actually one giant, primary relay) to deactivate the entire network in an instant.

This method worked for billions of years until the asari cycle, when prothean scientists of the previous cycle were able to hijack the command signal usually sent by the Reapers to their Keeper servants that would usually deactivate the relays, reprogramming the Keepers to instead ignore it. When Sovereign attempted to assume direct control of the Citadel itself, it was soundly defeated by Marcus L. Shepard, which removed the Citadel and the mass relays from Reaper control permanently. Large swaths of the mass relay network were temporarily disabled in 2186 CE following the firing of the Crucible, but were later restored to full operation. The mass relays continued in their function until 2480 CE, when a breakthrough in interstellar translocation was made. One by one the mass relays were decommissioned, until the last one was de-orbited in 2690 CE.

A mass relay was a massive structure of complicated engineering and sophisticated machinery. It was comprised of two, fifteen-kilometer long prongs, centered around a set of revolving, gyroscopic rings five-kilometers across. These rings encompassed the primary eezo reaction core, which in itself was built upon a self-producing element zero split-reaction principle, not entirely dissimilar from that of a harnessed, artificial star. Built from the same material as the Citadel, a mass relay was incredibly durable, to the point of being dubbed 'impossible to destroy.' This myth of indestructibility was maintained for billions of years, largely due to phenomena such as supernovas merely shifting the relay out of position without any damage imposed upon its superstructure. However, in 2185 CE, the myth was proven wrong when Marcus L. Shepard slammed an asteroid into the Alpha Relay, destroying it. Proving many theories, the destruction of a mass relay is an explosive, violent event: the force of an element zero centrifuge rupturing leads to a detonation of all the eezo inside, which leads to a "supernova level event" in which the explosion consumes everything within 350,000 AUs.

The hull of a mass relay also masks any heat signatures within, leading many sensors to register it as a 'cold' object in space: this fact makes it extremely difficult to detect or locate mass relays, especially if they shift position: this exact reason is why the Mu Relay wasn't found until 2183 CE.