In the present paper, a qualitative research of the cognitive skills of experienced software developers is presented. The data for the research was gathered using the Delphi method. The respondents were 11 software developers who have worked at least five years after their graduation. Two questionnaire rounds were conducted. In the first round, the respondents mentioned altogether 32 different skills. In the second round, 10 of the respondents answered and evaluated the difficulty of these 32 skills (e.g., does the skill efficiently differentiate experts from novices). The results are divided into two categories: composition and comprehension. Approximately 40% of the skills were classified into the category ``comprehension.'' For each skill, the evaluated degree of difficulty of the skill is presented. In the category comprehension, skills related to comprehension of a program or a problem as a whole were evaluated as the most difficult.