Introduction -- Characteristics of Particulate Matter Considering Particle Mass and Particle Number -- The Process of Formation of Particulate Matter in Combustion Engines -- Methods of Measuring Particulate Matter Emissions -- Particulate Matter Emissions during Engine Start-up -- Determination of Particulate Matter Equivalents -- Measurements of Particle Mass and Particle Number in Real Traffic Conditions -- The Relationship between Particle Mass and Particle Number -- Methods of Decreasing Emissions of Particulate Matter in Exhaust Gas -- Conclusions.