Introduced in the mid 19th Century, The Knowledge requires drivers to memorise 25,000 streets and 100,000 landmarks within a six-mile radius of Charing Cross. Successful applicants have to pass up to a dozen interviews with the entire process taking up to four years.

The London taxi test, formally known as "The Knowledge," is widely considered the most difficult and prestigious taxi qualification in the world. To earn a "Green Badge," an aspiring driver must memorize every street, landmark, and point of interest within a six-mile radius of Charing Cross—which encompasses approximately 25,000 individual streets and over 20,000 landmarks. The average student spends three to four years of intense study, often navigating the city on a moped to build their mental map. The testing process involves a series of increasingly difficult oral "appearances" with an examiner, where the student must "call" the shortest route between two obscure points from memory, describing every turn and junction along the way. Research has shown that successfully "The Knowledge" actually causes physical changes in the brain, specifically an enlargement of the posterior hippocampus, which handles spatial memory. In 2026, despite the ubiquity of GPS, the requirement remains just as strict, as it ensures that London's black cab drivers remain the "gold standard" of urban navigation, capable of bypassing traffic and road closures without ever needing a digital map.