Magnesium alloys have gained increasing interest in recent years for their potential applications in the automotive, aerospace, and electronics industries. Many of these alloys are strong due to solid-state precipitates formed through age-hardening. However, the strength of magnesium alloys is still significantly lower than that of aluminum alloys. Improving the strength of magnesium alloys requires a better understanding of precipitate structure, morphology, orientation, and their effects on strengthening and microstructural factors that control precipitate nucleation and growth. This review discusses precipitation in most precipitation-hardenable magnesium alloys and its relationship with strengthening. It is shown that precipitation phenomena in these alloys, especially in the early stages, are not well understood, and many fundamental issues remain unsolved despite extensive research over the past 12 years. Challenges in precipitation hardening and age hardening are identified, and guidelines are provided for the rational design and development of higher strength, and ultimately ultrahigh strength, magnesium alloys through precipitation hardening. Magnesium is the lightest of all structural metals, with a density about two-thirds that of aluminum. It is abundant and has a low melting temperature and high specific heat, making it suitable for casting. Despite its advantages, magnesium alloys have limited adoption in engineering applications compared to aluminum alloys due to limited options and insufficient properties such as yield strength, creep resistance, formability, and corrosion resistance. Age hardening, which involves solution treatment, quenching, and subsequent aging, is used to achieve mechanical properties in many magnesium alloys. The decomposition of a supersaturated solid solution often involves the formation of metastable or equilibrium precipitate phases that resist dislocation shearing. Controlling precipitation is crucial for achieving maximum precipitation strengthening. In Mg-Al alloys, the equilibrium solid solubility of Al in α-Mg decreases with temperature, and precipitation can produce a large volume fraction of precipitates. The precipitation process involves both continuous and discontinuous precipitation, with discontinuous precipitation occurring in grain boundaries and continuous precipitation inside grains. The orientation relationship between precipitates and the matrix is important for strengthening. The β phase in Mg-Al alloys has a body-centered cubic structure and can form plate or rod-shaped precipitates. The orientation relationship between β precipitates and the matrix is often near the Burgers relationship. In Mg-Zn alloys, the phase equilibria and precipitation are more complex than in Mg-Al alloys. The Mg7Zn3 phase has an orthorhombic structure and is thermodynamically stable at temperatures above 598 K. At lower temperatures, it decomposes into α-Mg and MgZn. The MgZn phase has a rhombohedral or monoclinic structure, and its orientation relationship with the matrix is important for strengthening. The precipitation sequence in Mg-Zn alloys is different from traditional understanding, and the structure and composition of precipitates are still not fully understood. In Mg-Zn-Al alloys, the addition of Al can enhance the age-hardening response.Magnesium alloys have gained increasing interest in recent years for their potential applications in the automotive, aerospace, and electronics industries. Many of these alloys are strong due to solid-state precipitates formed through age-hardening. However, the strength of magnesium alloys is still significantly lower than that of aluminum alloys. Improving the strength of magnesium alloys requires a better understanding of precipitate structure, morphology, orientation, and their effects on strengthening and microstructural factors that control precipitate nucleation and growth. This review discusses precipitation in most precipitation-hardenable magnesium alloys and its relationship with strengthening. It is shown that precipitation phenomena in these alloys, especially in the early stages, are not well understood, and many fundamental issues remain unsolved despite extensive research over the past 12 years. Challenges in precipitation hardening and age hardening are identified, and guidelines are provided for the rational design and development of higher strength, and ultimately ultrahigh strength, magnesium alloys through precipitation hardening. Magnesium is the lightest of all structural metals, with a density about two-thirds that of aluminum. It is abundant and has a low melting temperature and high specific heat, making it suitable for casting. Despite its advantages, magnesium alloys have limited adoption in engineering applications compared to aluminum alloys due to limited options and insufficient properties such as yield strength, creep resistance, formability, and corrosion resistance. Age hardening, which involves solution treatment, quenching, and subsequent aging, is used to achieve mechanical properties in many magnesium alloys. The decomposition of a supersaturated solid solution often involves the formation of metastable or equilibrium precipitate phases that resist dislocation shearing. Controlling precipitation is crucial for achieving maximum precipitation strengthening. In Mg-Al alloys, the equilibrium solid solubility of Al in α-Mg decreases with temperature, and precipitation can produce a large volume fraction of precipitates. The precipitation process involves both continuous and discontinuous precipitation, with discontinuous precipitation occurring in grain boundaries and continuous precipitation inside grains. The orientation relationship between precipitates and the matrix is important for strengthening. The β phase in Mg-Al alloys has a body-centered cubic structure and can form plate or rod-shaped precipitates. The orientation relationship between β precipitates and the matrix is often near the Burgers relationship. In Mg-Zn alloys, the phase equilibria and precipitation are more complex than in Mg-Al alloys. The Mg7Zn3 phase has an orthorhombic structure and is thermodynamically stable at temperatures above 598 K. At lower temperatures, it decomposes into α-Mg and MgZn. The MgZn phase has a rhombohedral or monoclinic structure, and its orientation relationship with the matrix is important for strengthening. The precipitation sequence in Mg-Zn alloys is different from traditional understanding, and the structure and composition of precipitates are still not fully understood. In Mg-Zn-Al alloys, the addition of Al can enhance the age-hardening response.