Baroque music is not only characterized but also classified by the dualism of stile antico and stile moderno. Though the definitions of dualism are different depending on theorists and composers, Monteverdi's theory and practice of prima prattica and seconda prattica are known more widely. From the view point of Monteverdi, it can be said that prima prattica is the contrapuntal style of Renaissance as a technique making music more important than the text and seconda prattica is the homophonic style making word more important than music. These two styles or techniques appeared throughout the Baroque period, but the Baroque period was opened by seconda prattica regarding stile moderno. Seconda prattica was beginning to appear in the Venetian school already at the end of Renaissance, but the embodment of it was realized by camerata of Italy at about 1600. They developed monody to depict the meaning of text clearly and made the new music forms finally come into the world, Seconda prattica became a technique creating new styles or forms that left the style of the previous period. Monteverdi did not make new music with seconda prattica, but his introduction of seconda prattica made a great contribution to the settlement and development of Baroque music. Since Monteverdi led the creative activity from the end of Renaissance to the middle of the Baroque period as we know, his works have all characteristics of prima prattica and seconda prattica. Especially, when we look over the 8 madrigal collections among his works, the first 4 collections were written with prima prattica and the last 4 colletions were written with seconda prattica. Monteverdi's seconda prattica attempts changes not to monophony but polyphony with the new styles and techniques such as homophony, chromatics, dialogue in monodic style and concitato style. Considering the above, Monteverdi could be considered a progressive with conservative inclination. Monteverdi's seconda prattica is understood also in the background of the conservative style. Seconda prattica was used in his madrigal collections(some parts of 4, 5, 6, 7, and 8). These are summerized as follows; (1) From the fourth collection, Monteverdi emphasized the meaning of text by means of monophonic texture and also employed polyphonic melisma for the expression of the text. Chromatics and dissonance do not appear frequently, but those being used are very expressive. Monteverdi showed a turning point of compositional technique for the music of the future. (2) Seconda prattica accepting unprepared dissonance began to appear from the fifth collection. Using the new technique of harmony and continue in madrigals, the new form known as contino madrigal was born. Monteverdi presented this for the first time when he put continue into the last 6 madrigals of the 5th collection. Another characteristic of seconda prattica found in the 5th collection is the continuo accompaniment, and concertato style by the combination and contrast of ensemble and chorus. (3) The 6th collection describes the meaning of the text by means of various musical devices. About half of the madrigals in the 6th collection are madrigal cycles. Lamento d'Arianna and Sestina expressing condolence to Martinelli are among the best known examples. (4) The 7th collection titled "concerto" is consisted of various madrigals for 2, 3, 4 and 6 voices including monody. The instrumental music for sinfonia, ritornello and concertato expression appears frequently. Each madrigal has a continuo melody and there are no madrigals written in the old style. (5) The 8th collection (Madrigali Guerrieri et Amorosi) is characterized by various expression as the title, "war and love" shows. For example, the expression of the first madrigal, Altri canti d' Amor is maximized by 6 voices, enlarged instrumentation and developed concertato style. Monteverdi partially employed concitato style as well as non harmonic tones more skillfully, and also triadic melodies and running repeated motives for the expression of terror, fury and warlike feelings. Monteverdi is the first composer who introduced the theory of Baroque dualism. He also explained the structure of Baroque music applying the theory of dualism to his madrigals. Monteverdi is a composer who lived in a transitional period between Renassance and Baroque, but he did not stay for only playing a bridge role to link both periods and showed a model for mature music employing dualism. Monteverdi not only introduced the theory of dualism but also transmitted sconda prattica to the various new techniques in his madrigals, making a great contribution to the development of dramatic works including opera and new music forms.