In the beginning of the jet age, the visibility of the exhaust from airplanes drew attention to the particles being emitted. Whether at altitude in a contrail or near the ground due to visible smoke, the particles in the exhaust made the aircraft engines' emissions visible for all to see. This visibility motivated studies to understand and control the particles being emitted by aircraft, and resulted in some of the first regulations on particle emissions represented by the on-going certification requirement of a smoke number measurement. Scientific understanding of both the measurement of particles and their impact on climate and human health have advanced considerably since then, and there is much active research to continue to better understand Particulate Matter (PM) emissions and their resulting impacts. Important landmarks will be presented that represent steps along the way from earlier interest in contrails and smoke trails to present day understanding of PM emissions and particle microphysics.