The ability to isolate intact, functional mitochondria from plant tissues is a key technique in the study of the genome, proteome, and metabolic function of the plant mitochondrion. Traditionally, mitochondrial plant researchers have turned to specific plant systems and organs (such as potato tubers and pea shoots) from which mitochondria are readily isolated in large quantities. However, increasingly, research is focused on a small number of model species, and there is a need to adapt existing protocols to allow the isolation of mitochondria from these model species. Arguably, the most important of these is Arabidopsis thaliana, for which a formidable array of genetic resources is available. However, because of its relatively small size and the absence of large heterotrophic organs, Arabidopsis is a challenging plant from which to isolate mitochondria. Here, we present two methods for isolating mitochondria from Arabidopsis, either from heterotrophic cell suspension cultures or from hydroponic seedling cultures. We also present details of commonly used assays to assess the physical and functional integrity of the isolated organelles.