Blue native gel electrophoresis is a native electrophoresis method that can be used for molecular weight determination for most soluble protein complexes as well as for most membrane proteins. Subsequent sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) can be used in a second dimension to resolve the complexes into their subunits. The method has been extensively used for the analysis of the respiratory chain complexes, for the determination of intermediates of mitochondrial protein import, and for the identification of the composition of the protein import machinery for mitochondria and chloroplasts. Here we describe the basic method and some applications in the research of mitochondrial protein import.