Joseph Robert Saddler (born January 1, 1958), known by his stage name Grandmaster Flash, is a Barbadian-American DJ. He created a DJ technique called the Quick Mix Theory. This technique serviced the break-dancer and the rapper by elongating the drum breaks through the use of duplicate copies of vinyl. This technique gave birth to cutting and scratching. It also gave rappers better music with a seamless elongated bed of beats to speak on. He also invented the slipmat.

He is the founder and creator of Grandmaster Flash and the Furious Five, the first rap group to be inducted into the Rock and Roll Hall of Fame in 2007. In 2019 he became the first hip hop artist to be honoured with the Polar Music Prize. On May 21, 2022, he acquired an honorary doctorate in Fine Arts from Buffalo State University. On June 1, 2023, he was awarded an honorary doctorate in music from Lehman College in the Bronx, NY. On August 4, 2023, Saddler was issued a proclamation from the city of New York stating that August 4 is Grandmaster Flash Day.^{[citation needed]}

Saddler's family immigrated to the United States from Barbados. He was raised in the Bronx, New York City, where he attended Samuel Gompers High School, a public vocational school. There, he learned how to repair electronic equipment. Saddler's parents played an important role in his interest in music. His father was a fan of Caribbean and African American recordings.

During his childhood, Joseph Saddler was fascinated by his father's record collection. In an interview, he reflected: "My father was a very heavy record collector.... I used to open his closets and just watch all the records he had. I used to get into trouble for touching his records, but I'd go right back and bother them." Saddler's early interest in DJing came from this fascination with his father's record collection as well as his mother's desire for him to educate himself in electronics. After high school, he became involved in the earliest New York DJ scene, attending parties set up by early luminaries like DJ Kool Herc and Disco King Mario.

Saddler's uncle Sandy Saddler was a featherweight boxing champion.

Grandmaster Flash observed the styles of the smooth transitions of a disco DJ versus the non-fluid non-BPM-matched transitions of early DJing. He chose to complete his studies with the former. Grandmaster Flash came up with a fingertip-to-vinyl and crossfader technique called the Quick Mix Theory. In doing this, he figured out a mathematical way to cut, paste, repeat, and extend a very minuscule piece of an existing composition. This innovation took place in two places in the Bronx. The first location was his parents' house, 2730 Dewey Ave in the Throggs Neck Projects of the Bronx. The completion of the Quick Mix Theory was done at 927 Fox Street in the Hunts Point Area of the Bronx.

The Quick Mix Theory gave birth to techniques such as cutting, scratching, and transforming.

In addition to Grandmaster Flash's on-time BPM-perfected technique, additional items were needed to pull this off. These items were felt and wax paper. The combination of these two materials cut into the size of a record and placed on the turntable (at that time referred to as a "wafer") allowed the record to move fluidly. The end result of that was the creation of what we today call the slipmat. Another item that played a key role in the successful implementation of the quick mix theory is the needle/stylus.