Secondary schools have experienced problems due to lack of adequate trained teaching staff, undisciplined students, inadequate physical facilities and failure to set educational objectives in curriculum and instruction delivery. These problems have jeopardized the head teacher‟s instructional supervision programmes in the concerned schools. The purpose of this study was to investigate the constraints faced by head teachers in instructional supervision in Matungulu, Machakos County, Kenya. The objectives of the study were: - (i) To investigate how secondary school head teachers carry out their instructional supervision practices.(ii)To assess the impact of effective instructional supervision on teachers in ensuring students performance in Matungulu, Machakos County.(iii)To find out the constraints faced by secondary school head teachers in undertaking instructional supervision. The study was guided by the theoretical framework based on motivation-hygiene theory proposed by Fredrick Hertzberg. The researcher adopted a descriptive survey design. The population of the study comprised 35 head teachers and 375 public secondary school teachers serving in the 35 secondary schools within the district. Out of the 35 secondary schools, 16 schools i.e. 46% were sampled for the study using stratified sampling technique based on day or boarding and single or mixed. Each of the 16 schools produced their head teacher yielding to 16 head teachers (46%). Similarly, for each of the 16 schools, one H.O.D and one teacher were sampled randomly yielding to 16 H.O.Ds and 16 teachers (9%).The overall sample size was 48 subjects .The research instrument comprised of the use of questionnaires for head teachers and teachers. The researcher conducted a pilot study to determine validity and reliability of the instrument in one secondary school in the district which was not included in the main study. He obtained permission from the MOE. The qualitative data obtained from the head teachers and teachers‟ questionnaire was analyzed by coding and organizing it into themes and categories. Once the themes and categories had been identified, the researcher then evaluated and analyzed the data to determine the adequacy of information in answering the research questions using frequency distribution tables, graphs and charts. The findings of the study were that despite their various degrees of success, each head teacher faces ongoing challenges in their instructional supervision work with regards to students‟ performance in National Examinations. The study recommended that head teachers‟ administrative skills should be improved in order to help them perform their functions effectively. The Ministry of Education should broaden and increase the frequency of in services courses to already serving head teachers. The courses should particularly emphasize on supervision of instruction. The deputy head teachers should also be included as they can take over supervisory functions from the head teachers or supplement the head teachers‟ role. Head teachers should make efforts to get all the teachers set their educational objectives and particularly at the beginning of the year in order to improve students‟ achievement of results and that the government should employ more trained teachers which will ease the supervisory roles of the head teacher in schools among other recommendations.