From the viewpoint of the sequence space, the protein folding problem can be regarded as a problem of searching for the determinants of protein foldability. The concept of “folding elements (FEs)” has emerged from the search for the amino acid sequence essential for a protein to be foldable. Here we review the characteristics of the FEs of a small globular protein, dihydrofolate reductase, and the necessary conditions for the protein to be foldable. We also review the relationship between the FEs and the kinetic folding reaction and propose the FE-assembly model of protein folding.