The Forerunner are shrouded in mystery. Although their cyclopean architecture still remains, they have long been absent from their own creations. What little we know of them is explained vicariously through their machinery and their automatons. Clearly, they were technologically superior to both humanity and the Covenant. But their allowance of the Flood's perpetuation could be considered an inexcusable lack of foresight for such an advanced civilization.

At one point in the Forerunner's history, they encountered the Flood. For whatever reason, they felt it was necessary to contain and study the Flood, rather than destroy them outright.

The Forerunner had abandoned the Halo some time ago, leaving their robotic compatriots behind to defend and monitor the installation. Seeing Master Chief and Cortana stand right athwart his plans to activate Halo, 343 Guilty Spark releases Halo's only known defensive system on them: the Sentinels. The Sentinels are floating robotic machines, using what is presumably Forerunner weaponry. Unlike the the human and Covenant weaponry, the sentinels used some sort of controlled stream of energy. Deadly and precise, they appear to be more like surgical cutting tools than the primitive and inaccurate weaponry used by their adversaries

But who were the Forerunner? The symbols depicted in the interiors of Halo's buildings are eerily similar to those seen on the backs of the Covenant and on their vehicles alike. But what of 343 Guilty Spark's comments of their lost history? Much remains to be answered about the Forerunner