Ionomics is the study of the elemental composition of biological tissues. It complements knowledge acquired by metabolomics, proteomics, bioinformatics, and genomics in elucidating the physiological status of plants as well as the identification of genes involved in the transport and metabolism of individual elements and their interactions. Inductively coupled plasma-mass spectrometry (ICP-MS) technology provides a very sensitive method for the medium- and high-throughput elemental analysis of plant tissues. This chapter introduces the plant physiologist to the ICP-MS technique, a method for sample preparation and analysis.