Recent studies on the (hypercoiled) compact conformation and the pH-induced conformational transition to the extended coil of the hydrophobic poly(acids or bases), which have been well used in industrial and medical circles, are summarized, especially on the basis of our studies on poly(alpha-alkyl acrylic acids) and maleic acid copolymers with hydrophobic aromatic monomers. The stability of the compact conformation and values of various thermodynamic parameters of the transition of the hydrophobic polyelectrolytes suggest the importance of hydrophobic interaction among the side chains to form the compact conformation. Characterization of various physicochemical methods, including 1H-NMR measurements, gives a model of the compact conformation with a more expanded structure than in globular proteins, a free-draining property of the solvent, the conformation fluctuation, etc. Also, the apparent two-state character of each segment in the transition region, a life-time in each state and the cooperative parameter for the compact form formation are discussed. A review of our conformational studies on the hydrophobic-hydrophilic polypeptides is also given. For the anion-induced conformation transition in basic homopoly(alpha-amino acids), hydrophobic characters of the anion and the side chain in the polypeptide are shown to be important, especially in terms of solvation. The difference in the induced conformation by the anion between random and alternating basic copolypeptides is explained by introducing a model which shows the importance of the sequence of hydrophobic and hydrophilic residues in the polypeptide to induce the ordered conformation of the anions. Also, we attempt to explain the difference between the induction of ordered conformation in the basic homo- or copolypeptide in reversed micelles with a large sulfonate (AOT) vs. that in aqueous AOT in terms of the hydrophobic character of the side chains in the polypeptides, AOT and the medium.