The aim of this dissertation was to gain a deeper understanding of how teaching in digital environments can be designed to give students opportunities to develop knowledge in two- and three-dimensional geometry. The dissertation consists of two sub-studies, the first using an ethnographic approach in which the researcher followed a teacher's teaching in a digital environment, in a Grade 9 class for a period of five weeks. In the first study, learning was defined based on reification theory, which describes that the understanding of mathematical concepts develops from operational to structural understanding. Data were collected through video-assisted observations, interviews and student tests. This material was analyzed with a focus on the ways the teacher used the digital tools to create mathematical situations that offered students opportunities to work with different representations and mathematical concepts. In the second study, characterized as educational design research,t he researcher along with two teachers designed teaching in two classes in Grades 8 and 9. The focus in this second study was shifted to how geometry teaching in digital environments can be designed to offer students the opportunity to develop their understanding of, and ways of interpreting, geometric figures. For support in designing lessons, Brousseau's theory of didactical situations, based on a constructivist approach to learning, was adopted. Data were collected through video-assisted observations and screen recordings of students´ work. This material was analyzed in two steps. First, the focus was on how students worked with the figures in relation to the assumptions made in the design. Thereafter, the attention was on properties of the designed environment that were considered to affect the students and to lead to changes in how they interpreted and used the figures when working with geometric tasks. The study resulted in five design principles that can serve as a guide for designing teaching in digital environments. Thus, taken together, the dissertation´s two sub-studies show that using digital tools in teaching gives the teacher not only more didactic variables to work with when creating lessons, but also, based on the needs that arise in teaching, more tools for shedding light on what some students may have difficulty detecting. Furthermore, the results show that digital tools can help in the creation of environments that stimulate students' way of examining, testing and reasoning about geometric figures and their properties, which are seen as important prerequisites for developing good knowledge in the field.