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Exposure to Particulate Matter (PM) has been associated with adverse health effects. The main objective of this study is to quantify children’s exposure to PM and the respective inhaled dose in Lisbon. For that, a time activity pattern survey was performed with the participation of 1189 children. In addition, PM was sampled inside 5 schools, 40 homes, 4 modes of transportation and in the respective outdoor environments. Time-activity pattern records showed that children spent 86% of their time indoors, especially at home and in the classroom. The PM2.5 and PM10 concentrations in classrooms (35.3 μg/m3 and 65.4 μg/m3, respectively) were more than double than in homes (14.5 μg/m3 and 18.2 μg/m3, respectively) and highly exceeded the limit values established by the Portuguese legislation for indoor air quality. The high indoor-to-outdoor concentration ratios (I/O) calculated in schools for PM2.5 (1.8) and PM10 (2.1) suggest that a substantial fraction of particles was generated by indoor sources. PM daily patterns for classrooms showed the importance of occupancy, resuspension of dust and cleaning activities for the elevated levels of particles. The average daily children exposure was 20.6 μg/m3 for PM2.5 and 31.5 μg/m3 for PM10. During weekdays, the classrooms contributed with 42% and 50% to the PM2.5 and PM10 daily exposure, and with 36% and 41% to the PM2.5 and PM10 inhaled dose, respectively. This work quantitatively demonstrated that indoor microenvironments (MEs) are the main contributors to personal exposure to PM and respective inhaled dose.