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Six chambers, Chambers 13 to 18, have associated Challenges that you can attempt after completing the normal game. The layout of the chamber is the same as the normal one, but you are invited to complete the chamber using, respectively, the least number of portals, the least number of steps and the least elapsed time. According to how well you do this, you are awarded a bronze, silver or gold medal for each challenge in each chamber. If you get the bronze medal for all eighteen challenges (six chambers with three challenges each), you unlock the Basic Science Achievement. With all eighteen silver medals, you get the Rocket Science Achievement and with all gold medals the Aperture Science Achievement.

Walkthroughs of the challenges are included in the individual Chamber pages. There, you can study them in the context of the normal chamber solutions. The links below take you directly to the challenge sections of those pages and to the relevant sections of the tips and advanced techniques page:

Least Portals[edit]

Least Steps[edit]

Least Time[edit]