In many countries, the role and functioning of schools are changing and so is what is expected of teachers in relation to Professional Development. This is in fact seen in the innovations and researches done globally in the changing world. The purpose of this study was to critically assess the Professional Development of teachers in relation to the administration of Head teachers in public primary schools in Trans-
Nzoia County. The County has been facing mass exodus of teacher to other professions, conflicts were on increase with the head teachers and KCPE results have been dwindling over the past four years from 2014 to 2017.The objectives of the study were to establish the nature and types of tools used for Professional Development of teachers in public primary schools Trans-Nzoia County, find out the level and intensity of teachers Professional Development, determine the challenges and impact of teachers‘ Professional Development and lastly identify the possible solutions to Teachers Professional Development. The study adopted Social System Theory by Theodore Shultz (1987) which gave a deeper insight to the research study. The study adopted descriptive survey design which was suitable for this study. The target population was 286, consisting of 25 Head-Teachers and 250 Teachers in public primary schools and 11 Education Officers. Simple random sampling technique was used to get a sample size of 145 respondents from all the categories. Both qualitative and quantitative data analysis techniques were used to analyze the collected data which was then presented through calculated percentages which were presented in form of pie-chats, frequency tables and graphs. Social Package for Statistical Science (SPSS) Version 21.0 was used to analyze data. The findings were presented through descriptive statistics by use of frequencies, tables, graphs and charts. The study found out that most of the teachers preferred part-time Professional Development and short courses than full time. About 30% of the teachers were participating or were for idea on Professional Development. The teachers who participated in Professional Development improved the performance of students tremendously and lastly all the teachers had a common challenge in relation to availability of finance and time factors which determined greatly in Professional Development. In conclusion both the Head teachers and teachers have done very little in Professional Development leading to mass exodus, conflicts and poor performance of pupils in sub-sequent years. The study recommends that the Teachers Service Commission constantly reviews its policies to accommodate the changing needs of teacher Professional Development, establish programmes that accommodate all teachers and should be affordable, TSC should review teachers‘ salaries regularly to motivate teachers to undertake Professional Development, All education stake holders (Ministry of Education, TSC, school communities, parents) should be encouraged to support Professional Development programmes involving teachers. A further study in Professional Development was suggested upon completion of this research study in other institutions of learning.