PM is a widespread air pollutant, consisting of a mixture of solid and liquid particles suspended in the air. Commonly used indicators describing PM that are relevant to health refer to the mass concentration of particles with a diameter of less than 10 μm (PM10) and of particles with a diameter of less than 2.5 μm (PM2.5). PM is a mixture with physical and chemical characteristics varying by location and season. Biological components such as allergens and microbial compounds are found in PM. Particles can either be directly emitted into the air (primary PM) or be formed in the atmosphere from gaseous precursors (secondary particles). Primary PM and the precursor gases can have both man-made (anthropogenic) and natural (non-anthropogenic) sources. The evidence on airborne particulate matter (PM) and its public health impact is consistent in showing adverse health effects at exposures that are currently experienced by urban populations in both developed and developing countries. The range of health effects is broad, but are predominantly to the respiratory and cardiovascular systems. This talk is to introduce the characteristics and distributions of PM in the ambient air and the recent progresses of health studies related to PM exposures.