Intrinsically disordered proteins (IDPs) belong to the newly discovered and still growing group of proteins. In contrast to globular proteins IDPs fail to fold into a well-defined tertiary structure under physiological conditions and they are characterized by extraordinary structural flexibility and plasticity. These features enable IDPs to adopt different conformations in response to different stimuli or different partners. Additionally, a pliable polypeptide chain, much more accessible in IDPs, causes that IDPs can undergo extensive posttranslational modifications which might lead to further modulation of IDPs conformation enabling rapid regulation of IDPs activity. In this way IDPs are involved in regulation of various regulatory pathways and promoting the assembly of supramolecular complexes. IDPs discovery reveals a new face of proteins and constitutes a new challenge for modern biochemistry.