This project is a participatory action study. The objective was to develop two related systems, where the Tambon Health Promoting Hospital is the core of implementation comprising of the heat-health warning system and the heat-related illness surveillance system. The development of the heat-health warning system was based on a concept proposed by the World Health Organization, namely: 1) Weather forecasting using data from the Meteorological Department; 2) Setting the threshold temperature based on the method from the study of Phongtape Wiwatanadate (2016) under the project “The Development of Health Warning System and Mechanism for Heat-Related Illnesses” and analyzing the data obtained from the Nakhon Sawan Provincial Health Office for the past 5 years and 3) Issuance of watch/warning messages to related persons including identifying vulnerable groups through participation from the community and all stakeholders in every step. Nonetheless, this project has added a fourth step of activity, namely monitoring, providing advice and first aid to vulnerable groups, carried out by village health volunteers in collaboration with caregivers. Development of both passive and active heat-related illness surveillance systems was also carried out at the target sub-district level. It was adapted from a surveillance model developed by the Centers for Disease Control and Prevention (CDC), which has four steps: 1) data collection, 2) data analysis, processing and interpretation, 3) summarizing the surveillance results and reporting the results of the surveillance to those involved, and 4) corrective action or prevention of problems. The duration of the project was 1 year and 6 months with the pilot area being Klang Daet Sub-district, Mueang District, Nakhon Sawan Province. The outcome of the heat-health warning system between 1 March-to 30 April, 2020 (2 months) showed that the warning level in the "warning" level (level 3) (temperature above 39 degrees Celsius) was the highest for 25 days and there were 28 cases of heat-related illnesses. Most cases were found at Moo 6, a total of 30 cases. The most common disease was heat syncope, totally 39 cases. No cases of heatstroke were found. For the heat-related illness surveillance system between March 1, 2020 and February 28, 2021 (1 year), it revealed that the warning level of "surveillance" (level 1) (temperature below 37 degrees Celsius) was the highest totally 280 days. Most cases were in March 2020, with 42 cases. Most cases were found at Moo 6, a total of 36 cases. The most common disease was heat syncope, with 47 cases. The ratio of female : male was 1.6:1. The top 3 most common age group were 65–69 years old, followed by 50–54 years and 60–64 years old, respectively. The top 3 risk factors were working outdoors or in a hot place, drinking tea or coffee, and having heart disease or high blood pressure, respectively. No patients with heatstroke were found. The overall project evaluation found that the sub-district agencys, such as the Tambon Health Promoting Hospital, together with the village health volunteers, were able to operate both systems by themselves without difficulty, not adding too much workload as it can be integrated with other activities that are routinely performed. Most importantly, there is almost no budget required for the activities, but it can prevent morbidity and mortality from heatstroke. Policy Recommendations Policy recommendations are divided into 3 levels as follows: 1. Provincial level. Nakhon Sawan Provincial Health Office should be the key to expanding the project area. It may begin with a meeting of the heads of district government agencies to establish policies for implementing the heat-health warning and the heat-related illness surveillance systems at least 1 sub-district per district in the first year and making an assessment to expand the project to other sub-districts in the following years. It can get the funding support from the Health System Research Institute. 2. National level. The Ministry of Public Health may issue the policy to implement the heat-health warning and the heat-related illness surveillance systems according to the guidelines presented in this report by piloting one sub-district in each province in the summertime for a period of 3-4 months, and then gradually expanding to cover the entire province. It may use the same policy implementation guidelines at the provincial level aforementioned. 3. Health System Research Institute (HSRI). It may present the results of the study in this project to the meeting of HSRI Board of Directors to propose to the relevant departmental administrators of the Ministry of Public Health, such as the Office of the Permanent Secretary, Department of Disease Control, Department of Health, Department of Medical Services, Department of Health Service Support, etc. in order to have policy-making and driving at the national level.