The import of a modest number of nucleus-encoded RNAs into mitochondria has been reported in species ranging from yeast to human. With the advent of high-throughput RNA sequencing, additional nucleus-encoded mitochondrial RNAs are being identified. Confirming the mitochondrial localization of candidate RNAs of interest (e.g., small noncoding RNAs, miRNAs, tRNAs, and possibly lncRNAs and viral RNAs) and understanding their function within the mitochondrion is assisted by in vitro and in vivo import assay systems. Here we describe these two systems for studying mitochondrial RNA import, processing, and functions.