Diesel PM is understood to comprise elemental carbonaceous particles, an organic fraction of soluble or volatile hydrocarbons and sometimes a sulfate fraction. The need to measure such diesel PM at very low levels and to measure it outside of the usual engine test laboratory makes it helpful to sharpen our understanding of this detail composition and how it comes about. Real time instruments for measuring soot and particle number concentrations make it possible to discern emissions levels much lower than filter based laboratory measurements, but an understanding of the relationships between these measurements and the historical reference methods makes them more useful for development and certification of engines. Efforts to use soot measurements in-use in order to meet NTE requirements have shown good correlation to the laboratory reference and have also provided some new information on the shortcomings of the reference methods. This paper will report on measurements made with a photo-acoustic soot instrument augmented by a filter gravimetric method, including results from tests with the laboratory reference method and those made in-use on vehicles. It references related work done decades ago, updating and refining what was learned then with observations made with today's more modern instrumentation, data collection and data processing.