Recent development in the physics of high-temperature superconductivity is reviewed, with special emphasis on the studies of the low-energy excitations of cuprate and iron-based superconductors. For cuprate superconductors, a phenomenology based on coexisting competing orders with superconductivity in the ground state of these doped Mott insulators is shown to provide a consistent account for a wide range of experimental findings. In the case of iron-based superconductors, studies of the low-energy excitations reveal interesting similarities and differences when compared with cuprate superconductors. In contrast to the single-band cuprate superconductivity with an insulating parent state, the ferrous superconductors are multi-band materials with a semi-metallic parent state and exhibit two-gap superconductivity when doped. On the other hand, both systems exhibit strong antiferromagnetic correlation and Fermi-surface distortion, leading to unconventional pairing symmetries with sign-changing order parameters on different parts of the Fermi surface. These findings suggest that the pairing potentials in both the cuprate and the ferrous superconductors are generally repulsive, thus favor a pairing mechanism that is electronically driven and a pairing strength that is closely related to the electronic correlation. The physical implications of the unified phenomenology based on antiferromagnetic correlations and remaining open issues associated with the cuprate and ferrous superconductivity are discussed.T c < T < T* [16,19,21] , and will be discussed in more details in Section 2.3. On the other hand, neither low-energy PG [7,17,35] nor Fermi arc phenomena [36] can be found in the electron-type cuprate superconductors in the absence of magnetic fields. Interestingly, however, break-junction tunneling spectra of a one-layer electron-type cuprate revealed PG phenomena at T < T c in the vortex state [37,38] . Similarly, spatially resolved scanning tunneling spectroscopic studies of the vortex state of the infinite-layer electron-type cuprate Sr 0.9 La 0.1 CuO 2 revealed PG features with a characteristic energy PG < SC inside the vortex core [7,10,17] . Moreover, electronic Raman scattering experiments on Nd 2-x Ce x CuO 4 also revealed contributions of an additional small energy gap in the SC state [39] . Thus, many of the seemingly puzzling asymmetric properties between the hole-and electrontype cuprates may be explained by the differences in the ratio of the SC energy gap ( SC ) relative to a competing order (CO) energy gap (V CO ) and by attributing the origin of the low-energy PG phenomena to the presence of a CO energy gap so that V CO ~ PG [7,[10][11][12][13][14][15][16][17] . Thus, the presence (absence) of the zero-field low-energy PG phenomena in the hole-type (electron-type) cuprate superconductors may be considered as the result of V CO SC (V CO < SC ), as schematically illustrated in Fig. 1(b). A feasible physical cause for such differences will be discussed later.In addition to the low-energy PG ...