Originating in Northern California's underground punk scene more than two decades ago, Green Day became one of the first punk bands whose music effectively entered into the mainstream culture. Although Green Day's biography has been documented in several books and the field of music analysis has expanded to include popular music, there has not yet been a detailed study of Green Day's music. The purpose of this study is to critically analyze five songs from Green Day's first album, 1,039 Smoothed Out Slappy Hours (1991) and five songs from their most recent album, 21st Century Breakdown (2009) to determine how the musical style of Green Day has changed over an eighteen year period. The study implements an adaptation of Jan LaRue's Guidelines to Stylistic Analysis, which examines sound, harmony, melody, rhythm, growth, and text. By comparing the analysis of the two albums, the study concludes that the musical style of Green Day's latest album became less confined to the historical notion of punk music.