A wide range of possibilities exist both in potential labelling approaches and in potential labelling meterials. In most labelling studies there is an intention to cause a shift in the 13C signature of the PLFAs extracted from soil and from this infer the sources of carbon being used by soil microbes. To be useful as a labelling materials it is necessary that the isotopic value of the label differs from that of the unlabelled material. Labelling can therefore be achieved using material which is either enriched or depleted in 13C compared to the unlabelled material. The scientific question being addressed should be the main driver for selection both the labelling material and the approach to be used.