On-line 3D worlds can make a significant contribution to K-12 education. More and more studies point to their pedagogic value and advocate their inclusion in the curriculum so as to support everyday educational practice in an innovative way. The studies focus on discovering ways of turning such environments into more efficient pedagogic tools. At the same time, there are many studies which seek ways of implementing effective online collaborative learning (Computer-Supported Collaborative Learning - CSCL) and they therefore investigate factors which influence this type of learning. There are many as yet unresolved research questions which necessitate further study and research. The present study focuses on the creation of an online 3D environment (CoSy_World) and the creation of suitable pedagogic conditions within this environment so that it can provide the basis for a well-structured collaborative learning experience for the subject of Mathematics in Primary Education. The current Doctoral thesis looks into student engagement in collaborative learning as a factor influencing its effectiveness. Student engagement in collaborative learning is explored through its three dimensions: a) behavioural engagement b) emotional engagement and c) cognitive engagement. There were two Experimental Interventions with students of the 5th Grade (first Intervention) and students of the 6th Grade (second Intervention) of a Primary School in Attica. Each intervention lasted 9 weeks and the students (represented by avatars) met online in the 3D CoSy_World four times in total. The research method used was a sequential exploratory mixed method through which we first collected qualitative data, out of which we then identified qualitative and quantitative criteria and indices in order to evaluate the results. For the data collection we made use of a range of sources such as chat messages from the software log files, videotaped recordings of the online meetings of the avatars in CoSy_World as well as the questionnaires distributed to students after each Intervention. The qualitative analysis was made on the basis of a range of methods and with regard to the statistical analysis we used non-parametric criteria. From the first intervention, it was evident that both the recommended CSCL script and the 3D CoSy_World played a significant role in enhancing student engagement in the process of collaborative learning in Mathematics. The second Intervention was carried out in order on the one hand to confirm the reliability and effectiveness of the 3D CoSy_World and the CSCL macro-script in enhancing student engagement in collaborative learning and on the other hand to look more deeply into the extent of the students’ engagement in the process of collaborative learning. The two Experimental Interventions and the comparison of their results have led to useful conclusions regarding the utilization of 3D CoSy_World in conjunction with appropriate pedagogies in the daily educational practice. They have also led to the formulation of useful conclusions for the research community focusing on related areas since the present study: a) Proposes the creation of specific pedagogic conditions within a 3D virtual learning environment focusing on the need for proper planning and the utilization of properly structured activities in 3D environments. b) Offers an innovative research approach which combines the application of a macro-script together with a 3D virtual world, while at the same time combining the positive features of 3D virtual learning environments with the advantages of collaborative learning as well as the basic tenets of Mathematical Education. In this way, it highlights the need for further research in relation to how a 3D learning environment can be utilized effectively in the context of collaborative learning. c) Attempts the creation of the online 3D CoSy_World and its utilization in the teaching of Mathematics in Primary Education, suggesting the need for further study and research of 3D virtual learning environments in the teaching of various subjects. In addition it makes recommendations about the educational design of 3D environments and presents empirical data related to the degree of student involvement in collaborative learning in the subject of Mathematics at Primary School level. Finally, it outlines a number of recommendations for further research.