This study is aimed at determining the validity and reliability of the assessment tool within the professional development of teaching staff. Qualitative and quantitative research methods were used in the study. For statistical analysis, the results of the 118 portfolio' assessment developed in mathematics and computer science were considered. 15 experts took part in the survey obtained by the Likert method. The Focus Group consisted of experts, who carry out assessments and trainers who carry out training. The assessment is based on the principles of validity, reliability, sufficiency and authenticity of the assessment. The validity of the teachers' portfolio assessment tool is analyzed in the context of meaningful, constructive, criteria-based evidence. The reliability of the tool is determined based on statistical analysis. Recommendations are presented, which is based on the development of valid and reliable assessment tools. The results of the study are valuable in the practice of experts in assessment.