The theoretical part focused on describing the characteristics of certain groups of children with special educational needs (SEN); defining the term 'inclusion' in the educational system; presenting the legal requirements in Slovenia and Croatia for successful implementation of inclusive practices; and describing the characteristics of integrating children with SEN into mainstream elementary schools. Finally, it defined the teachers' attitudes and their role in teachers' work and described the role teachers' fears play when working with children with SEN. The aim of the research was to investigate the attitudes of Slovenian and Croatian lower- primary teachers on integration of children with SEN into mainstream elementary school. In addition, the research aimed at investigating the fears teachers experience regarding the inclusion process. The participants were 116 lower-primary teachers (66 Slovenian and 50 Croatian) who completed an online questionnaire, designed specifically for this research. The research showed that Slovenian teachers faced most difficulties while working with children with intellectual disabilities, as well as with children with visual impairments. Croatian teachers, on the other hand, experienced most difficulties in the process of integration of children with autistic spectrum disorders and children with speech and language impairments. Slovenian teachers generally felt less competent to work with children with SEN than Croatian teachers did, while both groups of teachers felt most qualified to work with children with specific learning difficulties. Both Slovenian and Croatian teachers reported having to adapt their teaching methods to a great extent, with small differences between the two groups. The teachers mentioned they faced various challenges when teaching children with SEN, such as not being educated enough for teaching children with SEN and not being provided with the help of qualified expert associates in the school. The teachers do not generally consider teaching children with SEN to be difficult, but are afraid of working with certain groups of children with SEN, such as children with autistic spectrum disorders, children with intellectual disabilities, and children with behavioral and emotional difficulties. As opposed to Croatian teachers, Slovenian teachers feel more afraid of teaching children with visual and hearing impairments. In the process of integration of children with SEN into mainstream schools, Slovenian and Croatian teachers often feel left on their own. Since the role of lower-primary teachers is extremely important in the proces of children's education, they should be educated enough and should be able to feel competent to teach various groups of children.