Metabolomics, also known as metabonomics or metabolic profiling [1,2], is the study of the complement of small molecule metabolites present in cells, biofluids and tissues. Metabolites (biomolecules with masses below about 1500 Da) are the key molecules that sustain life, providing the energy and raw materials of the cell. As such, any disregulation or malfunction of the system, due for example to disease or stress, will usually cause changes in metabolite levels, which can be monitored by spectroscopic techniques. Metabolites form one component of the multilevel-systems approach to biology: they interact dynamically with macromolecules in a host of other levels and multiple spatial scales. For example, metabolite levels both regulate and are regulated by genes through the transcriptional and protein levels of biomolecular organisation, and in some cases may be instrumental in cross-species interactions, for example in symbiosis or parasitism. In contrast to proteins and nucleic acids, the metabolic profile is directly sensitive to the environment of the system, for example being strongly influenced by drugs and diet.In metabolomics, the aim is to obtain a global profile of metabolite levels with as little bias as possible, which is the so-called untargeted approach. This poses a severe challenge to analytical chemical procedures that are traditionally optimised for specific target molecules or chemical classes. In order to capture the widest set of metabolites, multiple chemical analytical technologies must be employed. Each technology results in large and complex datasets requiring extensive processing and statistical modelling to draw biological inferences. In this chapter we present some of the most widely used statistical techniques for analysing metabolomic data. We highlight the challenges, including those still to be addressed, and begin by describing the technologies used and how they influence the statistical characteristics of the data. In the first part of the chapter, we discuss the range of analytical tools used to acquire metabolic profiles and the associated characteristics of the data. In the second part, we focus on statistical approaches that can be applied independently of the analytical Handbook of Statistical Systems Biology, First Edition. Edited