In the modernizing world, computer science is not only a separate discipline for scientists but has an essential role in many fields. There is an increasing interest in developing computational thinking (CT) skills at various education levels – from kindergarten to university. Therefore, at the comprehensive school level, research is needed to have an understanding of the dimensions of CT skills and to develop a model for assessing CT skills. CT is described in several articles, but these are not in line with each other, and there is missing a common understanding of the dimensions of the skills that should be in the focus while developing and assessing CT skills. In this doctoral study, through a systematic literature review, an overview of the dimensions of CT presented in scientific papers is given. A model for assessing CT skills in three stages is proposed: i) defining the problem, ii) solving the problem, and iii) analyzing the solution. Those three stages consist of ten CT skills: problem formulation, abstraction, problem reformulation, decomposition, data collection and analysis, algorithmic design, parallelization and iteration, automation, generalization, and evaluation. The systematic development of CT skills needs an instrument for assessing CT skills at the basic school level. This doctoral study describes CT skills that can be distinguished from the Bebras (Kobras) international challenge results. Results show that wto CT skills emerged that can be characterized as algorithmic thinking and pattern recognition. These Bebras tasks were also modified to be used for setting directions for developing CT skills at the secondary school level. Eventually, a modified model for assessing CT skills is presented, combining the theoretical and empirical results from the three main studies.