They joined 147 countries and territories around the globe taking part in Earth Hour.
Landmarks such as Wrigley Field and Navy Pier turned off their lights as part of the global effort to highlight climate change.
At exactly 8:30 p.m., the Friendly Confines, joined the Willis Tower, the Merchandise Mart, and the Chicago Theatre in commemorating Earth Hour.
"Generating electricity causes the emission of greenhouse gases which leads to global warming and we know what that does to the environment," said Fidel Marquez, vice president of Com-Ed. "So this is a celebration of the environment and something to make people aware."
An international effort to raise collective awareness about climate change, Earth Hour is observed around the globe at 8:30 pm on the last Saturday in March, when people are asked to turn off all non-essential lights for exactly one hour.
And so from Sydney's Opera House to the Eiffel Tower in Paris to Germany's Brandenburg Gate, that's exactly what happened.
"Earth Hour started just five years ago in a single city, Sydney, Australia," said Jim Leape, general director of the World Wildlife Fun. "Tonight it is in more than 6,000 cities and towns and 147 countries around the world. It is a truly unique global event."
According to the World Wildlife Fund, which organizes the event, this year's Earth Hour will reach just about 1.8 billion people.
"I decided I'd participate," said Jessica Campion. "My family is in town, so we'll probably just hang out and chat and have a drink ... Don't need power."
"I'm the guy that saves my recyclables and goes out of my way to take them home so I like stuff like that," said Brian Bourne.
When it comes to conserving energy, many of us know the obvious, like using fluorescent light bulbs and keeping the thermostat a few degrees higher but Com-Ed's Fidel Marquez said sometimes we forget the little things that also add up
