Zlatko Ugljen's religious architecture represents an indispensable segment not only of the author’s creative work, but also of the overall contemporary catholic and islamic religious architecture in Bosnia and Herzegovina and in the region. Zlatko Ugljen designed more than 200 buildings in his career, and was presented with various awards both at local and international level. As a professor at the Faculty of Architecture in Sarajevo, he educated many generations of architects. However, the greatest contribution to the regional architecture Ugljen gave through his 28 projects for 29 religious buildings. Considering that Ugljen's work in religious architecture is, regardless of this fact, underrepresented in professional and scientific journals, the primary goal of this doctoral dissertation is to consolidate, analyze and evaluate Ugljen's realized and unrealized sacral projects. One of the objectives of this paper is to present them and place them into the context of both the world and regional architectural frameworks in order to get a clearer picture of the author's contribution and significance in contemporary architecture of Bosnia and Herzegovina. The intention of this paper is, inter alia, to systematize Ugljen's sacral oeuvre in chronological terms, then to research and analyse the documentation that is available concerning his sacral projects, as well as the articles published on those projects, and to offer an interpretation of the design features and point out to different circumstances and needs in which those designs were developed. Ultimately, the aim of the dissertation is to lay foundations for stylistic distinction between modernism and postmodernism in Ugljen's oeuvre, and to point out to significant influence of critical regionalism on his creative work . In the broad sense, the dissertation analyzes modern and postmodern religious architecture in Bosnia and Herzegovina (Catholic and Islamic) in social and political contexts that influenced its development, construction and deterioration. The dissertation is structured around a thematic-chronological framework. It is composed of an introduction, six sections and a conclusion. The introduction, set as the first section, is followed by the second section with a historical overview of modern architecture in Bosnia and Herzegovina and the region in the 20th century. The historical overview opens with an analysis of the development of the Bosnian style at the beginning of the 20th century, which marked the birth of modernism, the protagonists of which influenced Ugljen's understanding and creation of architecture. Furthermore, a brief insight follows into the history of architecture in Bosnia and Herzegovina between the two world wars. This period is presented more broadly in theoretical terms through the analysis of the history of architecture in Europe in the 1920s and 1930s through an overview of developments in architecture in the Kingdom of Serbs, Croats and Slovenes, respectfully, the Kingdom of Yugoslavia in that period. After overview of the European and regional architectural scene, special attention is devoted to the history of the architecture in Bosnia and Herzegovina between the two world wars, with information on the most important architects, buildings and stylistic features, followed by an insight into history of architecture in Bosnia and Herzegovina after the World War II. Considering that development of the prewar architecture was heavily influenced by the world and regional ideological and stylistic tendencies, this section focuses primarily on the world's most important theoretical guidelines, stylistic tendencies and architects. In the third section, the social and political events after the World War II in Yugoslavia and Bosnia and Herzegovina are discussed, because they directly affected the position of the Islamic Community and the Catholic Church, which commissioned Ugljen's projects. Furthermore, the dissertation provides an insight into the position of the Islamic Community and the construction of Islamic religious buildings in Bosnia and Herzegovina from the end of the World War II to the present, considering that the position of the Islamic Community directly and indirectly defined the conditions for designing mosques. Before moving to the analysis of stylistic features of the mosques in that period, the dissertation lists the ideas and guidelines for the construction of modern mosques in the second half of the 20th century in the world. The fourth section describes and analyzes Zlatko Ugljen’s life and career, followed by description and interpretation of his sacral projects. Sections 5 and 6 elaborate on the projects for realized and unrealized religious buildings of Zlatko Ugljen following the same methodological approach, meaning that projects are analyzed chronologically, by the year of their creation, divided along the two phases: projects made in the 20th century (Section 4) and those made in the 21st century (Section 5). The sixth thematic unit applies the same approach and deals with the projects created in the 21st century, when the architect in some works moved away from modernism as a stylistic expression and adopted contemporary stylistic tendencies following the idea of critical regionalism. After the analysis and interpretation of the design features of the religious buildings, the Section 7 focuses on the specifics of the stylistic expression of Zlatko Ugljen, analyzing the sources of Ugljen's specific architectural expression, which synthesizes modernism, as a basic stylistic starting-point, postmodernism and the influence of traditional architecture through the theoretical guidelines of critical regionalism. Furthermore, the sources and features of Ugljen's sacral opus are explained, taking into consideration the stylistic genesis and influences in his work, resulting in an analysis of the design features of the religious buildings. Considering that co-authoring was an important segment of Ugljen’s career, the paper also elaborates on his associates/co-authors and their influence on each other. The interiors for churches, the use of symbols and iconography, the treatment of the climate interrelation, the choice of materials and spatial concepts were also analyzed. The time determinant as a parameter according to which the sacral projects were analyzed was derived from the stylistic features of Ugljen’s projects. They were analyzed with regard to the placement of the realized or unrealized facility in a certain environment, the analysis being supported with an explanation of the needs and reasons for which the facilities were designed, as well as the historical context in which they were built or should have been built. In addition, Ugljen’s sacral projects are placed within the framework of religious architecture in BiH. This content is additionally enriched with excerpts from interviews with Ugljen's most important associates and co-authors (Nina Ugljen Ademović, Branko Tadić, Husejn Dropić and Fr. Marko Karamatić). The research is based on the heterogeneous materials, such as: encyclopedias, books, archival and project documentation, critical reviews, newsletters, notes, interviews, newspaper articles and photo-documentation, taken from the Archives of the Episcopal Ordinariate in Mostar, Franciscan Library in Mostar, Historical Archives in Sarajevo, Archives of the Riyasat of the Islamic Community in Sarajevo, the Matica Srpska Library in Novi Sad, the National and University Library in Zagreb and the City Library in Mostar. Significant documents were provided by the Archives of the Behram-Bey’s Madrasa and the Islamic Center in Tuzla, as well as by the Franciscan Parish Archives in Tuzla, Žeravac and Zabilje. These archives contain various documents that helped us to get more information and a better understanding of the processes of creation and construction of the facilities: conceptual or detailed design projects, work diaries, tenders, contracts, opinions of commissions, changes, offers, invoices and sketches. Design sheets for churches and mosques built before the 1990s, with the exception of St. Peter and Paul’s Church in Tuzla, are not available any more, as they were mostly destroyed during the war. The completeness of the processing of the material was affected by the fact that certain documentation was nonexistent, because if it had been available, it could have significantly supported the precise analysis of the proportions and other constructive values of these realized objects and thus contributed to the formal analysis of Ugljen's religious projects. The description and analysis of the construction of Ugljen's sacral projects presented in this paper, relying on projects and personal insights from the field research, can help further evaluation and research of his work in the future. Ugljen has won numerous domestic and international awards and some of its sacral buildings can be found in world-renowned publications on this topic. The religious architecture of Zlatko Ugljen belongs to the most praised, but also the most disputed parts of his work. Religious projects, built during the six-decade long creative period of Zlatko Ugljen, had different fates: two buildings were left unfinished, one was destroyed in the war, eighteen were realized, whereas eight projects remained unrealized. Thus, Ugljen's sacral opus is not large, but viewed within the history of contemporary architecture of Bosnia and Herzegovina, it has attracted the greatest interest of the world and regional public.