This thesis focuses on describing, understanding and improving telephone triage of patients with acute chest discomfort (pain, pressure or tightness) who call out-of-hours services for primary care (OHS-PC). (i) The Netherlands Triage Standard is used for telephone triage, but it performs poorly regarding both safety and efficiency in urgency assessment of patients with or without acute coronary syndrome or other life threatening events. Triage nurses’ overruling of the system recommended urgency increases the safety of telephone triage. (ii) Triage nurses create a ‘mental image’ of the patient to cope for the lack of visual information during telephone triage. ‘Congruence’ with the NTS is key for the working strategies of the triage nurses, defined as whether the system supports their decision making. (iii) The risk of an acute coronary syndrome among patients with chest discomfort is almost twice as high for patients calling at night (between 0.00-9.00 a.m.) as compared to patients who call at other hours; evidently in men, but less clearly in women. (iv) Symptoms predictive of an acute coronary syndrome are more similar than different for women and men with acute chest discomfort who call the OHS-PC. Ambulances are dispatched equally to women and men. (v) Among both women and men with chest discomfort, age, pressing/heavy chest pain, sweating and radiation of the pain are the strongest predictors for the diagnosis ACS.