In the 1890s, Jean Sibelius expressed his view concerning the potential that folk music and Kalevalaic runic songs in particular can offer to modern art music. In many of his works from the 1890s on, Sibelius sought alternatives to traditional tonality, functional harmony, and formal design of Western art music in modality of the Finnish folk-music tradition in a manner that led him to a new, unconventional, and highly individual approach to harmony and form—an approach that can be characterized as ‘modernist,’ although it drastically differs from the ‘modernism’ of many of his contemporaries, such as Schoenberg, Scriabin, or Stravinsky. The key elements of Sibelius’s new ‘Kalevalaic Modernism’ discussed in this chapter are modality and modal harmony, the construction of melodic material, and repetitiveness or ‘organic variation’ as a formal strategy. In the focus of the discussion are several works from the early 1900s to Sibelius’s last completed orchestral works.