Walter "Walt" Hartwell White Sr., also known by his clandestine pseudonym "Heisenberg", was a chemist and a former chemistry teacher in Albuquerque, New Mexico, who, after being diagnosed with inoperable lung cancer, started manufacturing crystal methamphetamine to both pay for his treatments and provide for his family in the event of his passing.

Once a promising chemist who greatly contributed to the breakthrough of a multi-billion dollar company Gray Matter Technologies, Walt abruptly left the company and sold his financial interest for $5,000. The founders of the company Elliott Schwartz and Gretchen Schwartz later married and made a fortune. Walt later harbors animosity and blames Elliot and Gretchen for stealing his hard labor and contributions to become a highly successful foundation, without giving Walt any credit. Walt then bitterly blames Gretchen for his financial problems and his lot in life.

Knowing nothing about the drug trade, he enlisted the aid of his former student, Jesse Pinkman, to manufacture and, more importantly, sell his meth. Walt's scientific knowledge and dedication to quality lead him to produce crystal meth of unrivaled purity. Walt eventually devised an alternative chemical process utilizing methylamine, giving his product a distinctive blue color. His crystal meth, christened "blue sky" by dealers and users alike, instantly began to dominate the market. While Walt was initially squeamish about the use of violence, he gradually came to see it as a necessity and eventually developed into a ruthless drug lord motivated largely by vanity, ego, and greed. That being said, his concern for his family and loyalty to Jesse remain mostly intact. As the series progresses, Walter gradually becomes darker and takes on a more villainous role.