Critical Mass is a monthly event that asserts bicyclists' right to ride in the streets and celebrates the bicycle as a form of transportation. The idea is for riders to take over city streets, with anywhere from a handful of cyclists to hundreds participating. In Oregon, the phenomenon is concentrated in the cities of Portland, Eugene, and Corvallis; however, rides have taken place in other cities, including Beaverton and Bend.

Since 1992, when the first Critical Mass ride was held in San Francisco, there have been similarly inspired rides that have attracted hundreds, and sometimes thousands, of riders in cities throughout the world.

The event has attracted significant controversy, particularly in Portland, where conflicts between police and participants have plagued Critical Mass rides. While police facilitated the first ride in Portland in 1993, the next year they arrested Critical Mass participants for blocking traffic. In 1995, Critical Mass organizers successfully sued the City of Portland after police stopped participants from distributing fliers without an event permit. Over the next several years, periodic conflicts between Portland's police and Critical Mass participants continued to flare up.

Shortly after taking office in 2005, Portland Mayor Tom Potter joined a Critical Mass ride, giving riders some optimism that city officials might unofficially embrace the event. In 2007, however, heavy police presence and the strict enforcement of traffic laws, including numerous arrests of riders, led organizers to cancel future rides. "Most people in the bike scene seem to have moved on," activist Jonathan Maus was quoted as saying in a 2007 Oregonian article.

While Portland's events garnered more attention in the media and among activists, Eugene's Critical Mass rides have often produced both enthusiastic turnouts and conflicts between participants and authorities. Rides in Eugene continue to take place on the last Friday of each month.