An important functional relationship of low temperature geochemistry is that there are coincidental shifts in the δ13C and δD of methane isotopic composition relative to the methane production pathway and/or to the effects of microbially mediated methane oxidation. This chapter aims to illustrate these shifts as they occur in terrestrial environments and to consider the factors that influence this relationship, including anthropogenic impacts (e.g., landfills). It shows that changes in 13CH4 alone are ambiguous, forced either by methane oxidation or variation in the methane production mechanism. Information on variation in δD of CH4 should strengthen any interpretation of 13C data. While C and H isotope systematics can be useful for diagnosing a wide variety of processes leading to methane production, this chapter focuses only on microbial methane produced or consumed in low temperature settings.