The origin of the organ instrument can be related to the location of Alexandria in present Egypt, where the first documents, connected to the prototype of today's organ instrument, were found. In contemporary organology there is a unique standpoint on the precise time of the emergence of the organ, as well as on the name of the creator. Therefore, it is considered that in Alexandria in 246 B.C. creator and mathematician Ctesibius constructed a machine he called pipe organ (hydraulis) – an auletic instrument which used water to push air toward the pipes, in order for them to produce sound. The development of the organ instrument and its popularity can be tracked during the entire Antique period, but also onwards during the Middle Ages in which around the 10th century, the organ became and instrument that found its place in churches; at first in large monasteries and cathedrals and later on in smaller churches. By taking over the dominant role in liturgies, all of a sudden the organ became a factor and co-creator of liturgy. Hence, the Catholic church at the Council of Milan in 1287 permitted that the organ officially be introduced into the liturgy and recommended its usage. With the entry of the organ instrument into the church, its popularity was increasing irreversibly, and the peak of its creative momentum appeared in the Baroque period. By looking at the development of the organ instrument in Europe, it can be concluded that three great organ schools existed: thus, one can differentiate between the French, the German and the Italian organ schools. Due to archival materials such as legal documents, visitation records or commissions, information on the oldest recorded organs in Croatia is available. The first record relating to the organ instrument in Croatia is connected to the memorial of organ player Nikola in St. Mark's Church in Zagreb in 1359. In Dubrovnik, the organ instrument is mentioned in 1384, in Zadar in 1392, in Split in 1412, in the Zagreb Cathedral in 1412 and the Krk Cathedral in 1500. The first data on the builder of the organ in Croatia refers to master Nikola Faller. The oldest preserved organ instrument in Croatia is in Lepoglava, built in 1649 by Johann Georg Freundt (cca. 1590-1667) from Klosterneuburg near Vienna. The name of the first professional organ builder Antonius Weiner (1706 - 1747) from Varaždin, appeared in the 18th century, whose preserved organ instrument today stands in the Franciscan church in Samobor (1722). During that period in the northern parts of Croatia there are organ instruments built by Ivan Juraj Eisl (cca. 1708 - 1780), a builder from Ljubljana in Marija Gorica (1759) and the exceptionally beautiful organ of the Austrian builder Antonius Römer (1724 –1779) in Trški Vrh. Organ music in Croatia has been developing since the 16th century, but it was solely associated with liturgy, primarily because of the collection of smaller instruments of limited ix technical and audible features. Instruments of the Italian organ school were found in the southern part of Croatia, on the Adriatic coast, while in the northern part of Croatia, instruments were predominantly constructed by Slovenian and only a few local pipe organ builders. The first large concert-type instrument was the pipe organ of the Hungarian builder Franjo Fucht (1779-1852) erected in the Zagreb Cathedral in 1834, but due to technical problems, the instrument was soon removed. Archbishop Juraj Haulik (1837-1869) commissioned a new grand instrument from the German builder Erberhard Friedrich Walcker (1794-1872) and it was erected in 1855. The first great composer and organ player of the Zagreb Cathedral was Franjo Dugan senior (1874-1948). The opus of Franjo Dugan senior became the fundamental platform and the determinant of activities of his successors, organ players of the Zagreb Cathedral - Franjo Lučić (1889-1972), Dugan's student and Anđelko Klobučar (1931-2016), Lučić's student. Dugan acquired his knowledge studying music in Berlin and passed on his experience to the domestic milieu, directing the activities of many Croatian composers toward the European trends of that time. Franjo Lučić held a prominent place among Dugan's students and took over the compositional practice of his teacher, but also went a step further, following modern tendencies that were developing during the 1930s. The creative personality of Anđelko Klobučar, who studied composition with Milo Cipra and the organ with Franjo Lučić, emerged on the wave of Modernism and new horizons. He continued his music studies in Salzburg and Paris and, being enriched with new ideas, refined his expression based on Lučić's practice. Through an analysis of selected works of Franjo Dugan Sr., Franjo Lučić and Anđelko Klobučar, our goal was to discover changes, the emergence and the termination, as well as the development of new trends, that changed former rules in the context of Croatian organ music and, on the basis of an analytical approach and comparative methods, collate the developmental principles of their composing. In that sense, after principles of elementary, functional and structural analysis of works, special emphasis has been placed on the following components of music forms: a) formative elements (rhythm, melody, harmony, polyphony, dynamics, agogics, timbre); b) proportions (the relations between wholes on micro and macro levels); c) the relation toward the breakdown of form and the inclination toward fragmentarity (treating and shaping fragments in relation to the whole); d) organ registration. The analysis and comparison of organ scores crystallised the basic characteristics i.e. similarities and differences in the composer's gestures, but also in the features of evolvement for every composer separately, with a goal to find and specify their mutual influences. x On one side it was possible to recognise certain analogies that were transferred from the teacher to the student. Thus, Lučić took the polyphonic expression from Dugan, while simultaneously sought new manners of expression and commenced to abundantly use the homophonic structure. Lučić began to use the organ more as an orchestra, and accordingly in a specific manner enriched the timbre of his compositions. Klobučar built on this rich tone approach of Lučić, but his expression was different since into his works, he introduced avantgarde processes that turned toward a more liberal relation to tonality, dissonance, metre and melody. Chronologically, their organ opuses follow one another in a special manner. Furthermore, they created an uninterrupted flow in the development of organ music over a period of more than one hundred years. Finally, an influence of European flows on the development of organ compositions of the three prominent Croatian composers and organ players has also been established. Walcker's pipe organ of the Zagreb Cathedral is the starting point of Croatian organ production and reproduction. It is the most significant and most beautiful instrument in Croatia that has been assembling musicians, not only organ players, but also numerous composers who, inspired by its sonorous timbre, dedicated a part of their opus precisely to organ works. The development of the organ in Croatia would have taken an entirely different direction, had the Zagreb Cathedral not been in possession of such a top-quality instrument and probably the opus would have been significantly more modest in the number of newly composed works. We shall never be able to sufficiently express our appreciation for this step forward, and for the farsightedness of Archbishop Juraj Haulik and the organ instrument of the Zagreb Cathedral.