We use a coarse grained protein model that enables us to determine the equilibrium phase diagram of natively folded α-helical and unfolded β-sheet forming peptides. The phase diagram shows that there are only two thermodynamically stable peptide phases, the peptide solution and the bulk fibrillar phase. In addition, it reveals the existence of various metastable peptide phases. The liquidlike oligomeric phases are metastable with respect to the fibrillar phases, and there is a hierarchy of metastability. The presented phase diagram provides a solid basis for understanding the assembly of polypeptide chains into the phases formed in their natively folded and unfolded conformations.