The rumen is a complex ecosystem essential for the health and productivity of ruminants. Metabolomics is an emerging technology, with rumen metabolomics research generating important data offering insights into the metabolites present in the rumen, and the factors affecting the rumen microbiome and metabolome. The importance this virtual organ plays in whole animal function is profound. With this, a greater understanding of the factors that impact the rumen composition and its functionality will be key to maximising health, productivity and sustainability of ruminants in the future. This chapter provides an overview of metabolomic analysis, the importance of the rumen metabolome and the factors that affect its composition and function.