So far we have discussed the emission and control of trace gases and particles. In this chapter we will look more closely both at how to sample them correctly and how to measure their concentrations. Trace gases, by definition, normally have concentrations in the ppb and ppt ranges, and therefore require more specialist techniques and sensitive methods than those in general use for gases such as water vapour or carbon dioxide that occur at percentage or ppm concentrations. Although filtration provides the simplest method of measuring particles, other techniques enable real-time measurements, or size distributions, or chemical analyses to be made. For both gases and particles, correct sampling is important to avoid corruption of the ambient concentration.