Headquartered in Issaquah, Washington, KCLS was the busiest library system in the United States as of 2010, circulating 22.4 million items.

KCLS offers a collection of more than 4.1 million items, including books, periodicals, newspapers, audio and videotapes, films, CDs, DVDs and extensive online resources.

[8] KCLS also extends reciprocal borrowing privileges to residents of many other library systems in Western and North Central Washington.

[3] The library eliminated its late fines in 2023 after finding it discouraged borrowing and cost more to collect and process.

[13] KCLS is subscribed to OverDrive, an online service that offers digital e-book, audiobook, and magazine checkouts for library patrons.