In 1963, four years after recording Kind of Blue, his most successful album to date, Miles Davis began to assemble a new ensemble to record and tour. But much had changed in those four years. Ornette Coleman's sensational 1959 premiere at the Five Spot Café in Manhattan's East Village introduced audiences to a free improvised "new thing" in jazz and marked the emergence of an avant-garde. In turn, critics quickly portrayed Coleman, along with Cecil Taylor, Archie Shepp, and others, as insurrectionists who were intent on shattering the jazz tradition. Meanwhile, jazz venues in New York shuttered as the city government implemented urban renewal programs that targeted vice districts for "slum clearance, " reshaping the city's topography and impinging upon its music culture. As the "new thing" gained increasing critical attention, and musical experimentalism among emergent and veteran improvising musicians flourished, the music of Miles Davis's "Second Quintet" gradually became more "free." This dissertation offers an explanation of this stylistic change via the experimentalism of the so-called jazz avant-gardists, tracing how "free" (i.e. timbral, chromatic, polymetric, and free meter) improvisation proliferated in their live and studio recordings up to 1968. I suggest that the increasing abstraction and volatility of the Quintet's music can be best understood in the context of the jazz avant-garde and the tumultuous social and structural changes of the 1960s. I index the stylistic change of the Quintet chronologically across four chapters. Chapter One discusses the emergent jazz avant-garde and New York City's changing jazz culture and infrastructure circa 1959. The first half of Chapter Two is an exegesis of avant-gardism in critical jazz literature; the second half of the chapter goes into detail on the improvisational and compositional techniques of the jazz avant-garde. Chapter Three explores three important events in the Quintet's timeline: the departure of tenor saxophonist George Coleman, the addition of Sam Rivers on the group's Japanese tour in the summer of 1964, and the Quintet's several-week engagement at Chicago's Plugged Nickel nightclub featuring Wayne Shorter in December 1965. Chapter Four begins with an overview of a revealing critic roundtable on the jazz avant-garde printed in a 1965 issue of Down Beat magazine that vividly illustrates the mostly negative reception of the "new thing" and the relatively narrow space that these artists had to respond to criticism. The latter half of the chapter shows the Quintet's transformation between 1966-67, comparing the growing abstraction and intensity in their music with that of Cecil Taylor's 1966 LP Unit Structures. By focusing on free improvisation and calling into question genre as reliable taxonomy for artistic praxis, I seek to provide deeper understanding of Miles Davis's music and that of the jazz avant-garde of the 1960s, acknowledging experimentation as an important pretext for innovation. Utilizing archival research, comparative music analysis, and original musician interviews, I show how expressive freedom was a shared ideal among improvising musicians across the musical field, from the underground into the mainstream. As such, experimentalism, I argue, is the link between the jazz avant-garde and Davis's quintet during a transformational moment in American music history.