Quantum criticality describes collective fluctuations in matter undergoing a second-order phase transition at zero temperature. Heavy-fermion (HF) metals have become key systems for studying quantum critical points (QCPs). These systems exhibit non-Fermi-liquid (NFL) behavior, such as T-linear resistivity, and show emergent low-energy excitations. Quantum criticality also interacts with unconventional superconductivity. HF metals, like CeCu₆₋ₓAuₓ and YbRh₂Si₂, exhibit QCPs through doping or magnetic fields. The Kondo effect, which involves spin fluctuations, plays a crucial role in these systems. Quantum criticality leads to the destruction of Kondo screening and the emergence of new critical modes. Theoretical studies have identified two types of quantum criticality in HF metals: one extending standard second-order phase transition theory, and another involving intrinsic quantum mechanical excitations. The quantum critical scaling regime is characterized by divergent critical exponents and a large entropy. The effective mass of charge carriers diverges at the QCP, and the Fermi surface may collapse. The interplay between quantum criticality and superconductivity is an active area of research, with evidence suggesting unconventional pairing mechanisms. HF metals provide a rich platform for studying strongly correlated electron systems, with implications for understanding quantum critical phenomena and superconductivity.Quantum criticality describes collective fluctuations in matter undergoing a second-order phase transition at zero temperature. Heavy-fermion (HF) metals have become key systems for studying quantum critical points (QCPs). These systems exhibit non-Fermi-liquid (NFL) behavior, such as T-linear resistivity, and show emergent low-energy excitations. Quantum criticality also interacts with unconventional superconductivity. HF metals, like CeCu₆₋ₓAuₓ and YbRh₂Si₂, exhibit QCPs through doping or magnetic fields. The Kondo effect, which involves spin fluctuations, plays a crucial role in these systems. Quantum criticality leads to the destruction of Kondo screening and the emergence of new critical modes. Theoretical studies have identified two types of quantum criticality in HF metals: one extending standard second-order phase transition theory, and another involving intrinsic quantum mechanical excitations. The quantum critical scaling regime is characterized by divergent critical exponents and a large entropy. The effective mass of charge carriers diverges at the QCP, and the Fermi surface may collapse. The interplay between quantum criticality and superconductivity is an active area of research, with evidence suggesting unconventional pairing mechanisms. HF metals provide a rich platform for studying strongly correlated electron systems, with implications for understanding quantum critical phenomena and superconductivity.