While access to essential medicines has become recognised as a human right, problems concerning the inappropriate use of medicines have emerged. Medicines lose their therapeutic value and can impair both individual health and public health if inappropriately used. Inappropriate use of medicines diminishes the quality of healthcare and causes resources to be wasted, which is especially serious in under-resourced countries. To improve the use of medicines, the introduction of standard treatment guidelines (STGs) is suggested as a potential strategy. The aim of this DrPH thesis is to study the use of medicines, focusing on adherence to new STGs in Community Health Centers (CHCs) in Timor-Leste. The country is now trying to establish an equitable and sustainable healthcare system under extremely resource-limited conditions. The study used mixed research methods, collecting data from randomly selected 20 rural CHCs, 1,799 retrospective samples from patient registration books, 583 prospective observations, and 55 semi-structured interviews of health personnel were collected. Timor-Leste's medicine use was found, in general, to be acceptable. For example, use of injections was extremely low. Training, especially clinical nurse training, influenced knowledge of, attitudes to, and practical use of medicines and also prescribing adherence to STGs. Other factors that influenced the use of STGs were: health personnel's agreement with the policy concept and contents of STGs; health personnel's positive perception of the changes brought about by the introduction of STGs; development of STGs in a health policy framework: the fact that their STGs were easy to use repeatedly; and a supportive environment and systems to use STGs. Constant socialisation and timely updates of STGs are necessary. Interrelation and consistency across policies and programs should be maintained. The position of training within the health policy framework should be clearly understood by the all people concerned. Follow-up supervision is needed both for individuals and the CHC. Anticipated support should be well functioning.