Walter Hartwell "Walt" White Sr., also known by his alias Heisenberg, is the main character of the American crime drama television series Breaking Bad. The character is portrayed by Bryan Cranston. Although AMC officials initially hesitated to cast Cranston due to his previous comedic role on Malcolm in the Middle, Gilligan cast him based on the actor's past performance in The X-Files episode "Drive", which Gilligan wrote. Cranston contributed greatly to the creation of his character, including Walt's backstory, personality, and physical appearance.

Within the series, Walt is a chemist and graduate of the California Institute of Technology, who co-founded the company Gray Matter Technologies. He left Gray Matter abruptly, selling his shares for $5,000. Soon afterward, the company made a fortune, much of it from Walt's research. Walt subsequently moved to Albuquerque, New Mexico, where he became a high school chemistry teacher. On his 50th birthday, he is diagnosed with Stage IIIA lung cancer. After this discovery, Walt resorts to manufacturing and selling methamphetamines with a former student, Jesse Pinkman (Aaron Paul), to ensure his family's financial security after his death. He is pulled deeper into the illicit drug trade, becoming more and more ruthless as the series progresses, and later adopts the alias "Heisenberg", which becomes recognizable as a kingpin figure in the Southwestern drug trade. Walt becomes less sympathetic throughout the show, as series creator Vince Gilligan wanted him to turn from "Mr. Chips into Scarface".

Both the character and Cranston's performance have received critical acclaim, with White frequently being mentioned as one of the greatest and most iconic television characters of all time. Cranston won four Primetime Emmy Awards for Outstanding Lead Actor in a Drama Series, three of them being consecutive, becoming the second actor to do so, after Bill Cosby for I Spy in the 1960s. Following his fourth win, Cranston tied Dennis Franz for the most wins in the category's history. He is the first man to win a Critics' Choice, Golden Globe, Primetime Emmy, and Screen Actors Guild Award for his performance.