Aperture Science (which is the firm that developed the Portal gun) is in direct competition with Black Mesa (which is the firm for which Gordon Freeman worked) for military contracts
See the section Other facts on this link Aperture Science and Black Mesa were bitter rivals.
As seen in the Portal maps "testchmb_a_15" and "escape_02", Aperture Science employees were briefed on Black Mesa through slideshow presentations, such as one titled "Dollar$ and Sense: Competing with Black Mesa for DoD and Government-wide Acquisition Contracts" EDIT : Here's an excellent guide on Half Life: http://members.shaw.ca/halflifestory/ Check out the timeline for an awesome detail of all events in the Half Life saga, and in the bottom a little speculation to its relation to Portal: The date that the events of Portal take place aren't certain.
We do know that GLaDOS says to Chell: "Are you trying to escape?
[chuckle]
Things have changed since the last time you left the building.
What's going on out will make you wish you were back in here.
I have an infinite capacity for knowledge, and even I'm not sure what is going on outside.
All I know is I'm the only thing standing between us and them.
Well, I was.
Unless you have a plan for building some supercomputer parts in a big hurry, this place isn't going to be safe much longer."
This implies that the game takes place during the Portal Storms or afterwards.
This site is not official but it's a great analysis IMO.
gaming.stackexchange.com/users/14/juan
In the original Portal, just before the chamber containing GlaDOS, there is a projector with some slides detailing that Black Mesa and Aperture Science are in direct competition.
In the song at the end of Portal , GlaDOS mentions Black Mesa: "Maybe you'll find someone else to help you.
Maybe Black Mesa...
that was a joke, ha-ha, fat chance" In HL2:Ep2, there are references to the Borealis, an Aperture Science research boat that went missing.
In Portal 2 (which takes place in an Aperture facility) , they mention a boat-yard where some sort of "time-travel experiment" went wrong.
Later, you find an empty ship-port , with life-savers scattered around that say "Borealis."
This concept art for HL2:Ep3 contains the Borealis.
In Portal 2, Cave Johnson (CEO of Aperture Science) states his hate towards Black Mesa in a recording , claiming they "stole their technology" from Aperture Science.
In Portal 2, (just before you are first introduced to the goo)
there is a room in Aperture Science with a bunch of trophies and awards for "#2 Applied-Science company of the year," implying (along with the projector slides from Portal 1) that the #1 company is Black Mesa.
gaming.stackexchange.com/users/2578/BlueRaja - Danny Pflughoeft
This page has a timeline of the rivalry between Black Mesa and Aperture Science
In HL2 Ep 2, You are told about an aperture ship called Borealis , it is suggested that this ship will appear in episode 3.
So both games take place in the same universe, and Valve seem to be intertwining the two stories.
There has also been rumour of a portal gun appearing in episode 3
gaming.stackexchange.com/users/92/Neil Aitken
In Portal 2, when Chell gets out at the end, it shows the outside for a brief moment and it almost looks like this must take place after the Half Life saga and after the Apocalypse.
There is nothing but the Aperture Science lab, so i think Aperture Science must be the Combine's; if it wasn't, they would have destroyed it, right?
gaming.stackexchange.com/users/15798/Jonathan
At the end of Half-Life ep 2
the borialis has a cargo contaner thith th aperture logo on it
I replayed the game and saw it again also near the end they reply to go to th borialis
gaming.stackexchange.com/users/83216/user83216
I found something extremly cool!
One day i went on the internet and then went on YouTube
, i wached a videa about Cell and the fan made theres about her and
even though i have played Portal 2 it was the first time i saw the picture of caroline.
The next day i bought Half Life 2 Ep 1 and 2 and saw the video clip of the ship that was apertures ship
and i was stunned that and looked for news for Ep 3
adn
then i hade a brainstorm... Search for "Family Picture half Life 2" and "Caroline portal 2"
See anything alike?
gaming.stackexchange.com/users/50645/Alex
I heard a lot of rumors about Portal series and Half-Life series plots relation but I didn't found any link.   Can you describe which relations exists between this two stories?
What are the connections between Portal series plot and Half-Life series plot?
gaming.stackexchange.com
<half-life-series><portal-series>
gaming.stackexchange.com/questions/122
gaming.stackexchange.com/users/730/Drake
Both games take place in the same universe. In the original Portal, just before the chamber containing GlaDOS, there is a  projector with some slides  detailing that Black Mesa and Aperture Science are in direct competition.  In Portal 2, when Chell gets out at the end, it shows the outside for a brief moment and it almost looks like this must take place after the Half Life saga and after the Apocalypse. Both games therefore take place in the same universe, and Valve seem to be intertwining the two stories.
In the original Portal, just before the chamber containing GlaDOS, there is a  projector with some slides  detailing that Black Mesa and Aperture Science are in direct competition.   Both games therefore take place in the same universe, and Valve seem to be intertwining the two stories. In Portal 2, when Chell gets out at the end, it shows the outside for a brief moment and it almost looks like this must take place after the Half Life saga and after the Apocalypse.