Relativistic effects play a crucial role in understanding the reactivity of gold(I) complexes in homogeneous catalysis. These effects, particularly the contraction of the 6s orbital and expansion of the 5d orbitals, significantly influence the electronic structure and reactivity of gold. This review discusses the experimental and theoretical insights into gold catalysis, highlighting the unique properties of gold(I) complexes, such as their strong Lewis acidity and ability to stabilize cationic intermediates. Theoretical studies, including natural bond orbital (NBO) analyses, support the idea that the relativistic contraction of the 6s orbital contributes to the strong Lewis acidity of gold(I) and its ability to activate alkynes for nucleophilic addition. Gold(I) complexes are particularly effective in catalyzing a variety of reactions, including hydroarylation, hydroamination, and cycloisomerization of enynes. The reactivity of gold(I) is attributed to its ability to form carbenoid intermediates and its unique electronic structure, which allows for efficient activation of alkynes and other π-systems. Theoretical calculations also suggest that the 5d orbitals of gold are more diffuse and less involved in backbonding compared to other transition metals, which influences the reactivity and selectivity of gold catalysts. Relativistic effects are also important in understanding the behavior of gold in various catalytic cycles, including oxidative addition and reductive elimination. The electronic structure of gold, influenced by relativistic effects, allows for the formation of stable cationic intermediates and the efficient activation of substrates. The review also discusses the development of new methodologies and the potential of gold catalysts in creating novel reaction pathways. Overall, the combination of experimental and theoretical studies provides a deeper understanding of the unique properties of gold in catalysis, highlighting the importance of relativistic effects in the reactivity and selectivity of gold complexes.Relativistic effects play a crucial role in understanding the reactivity of gold(I) complexes in homogeneous catalysis. These effects, particularly the contraction of the 6s orbital and expansion of the 5d orbitals, significantly influence the electronic structure and reactivity of gold. This review discusses the experimental and theoretical insights into gold catalysis, highlighting the unique properties of gold(I) complexes, such as their strong Lewis acidity and ability to stabilize cationic intermediates. Theoretical studies, including natural bond orbital (NBO) analyses, support the idea that the relativistic contraction of the 6s orbital contributes to the strong Lewis acidity of gold(I) and its ability to activate alkynes for nucleophilic addition. Gold(I) complexes are particularly effective in catalyzing a variety of reactions, including hydroarylation, hydroamination, and cycloisomerization of enynes. The reactivity of gold(I) is attributed to its ability to form carbenoid intermediates and its unique electronic structure, which allows for efficient activation of alkynes and other π-systems. Theoretical calculations also suggest that the 5d orbitals of gold are more diffuse and less involved in backbonding compared to other transition metals, which influences the reactivity and selectivity of gold catalysts. Relativistic effects are also important in understanding the behavior of gold in various catalytic cycles, including oxidative addition and reductive elimination. The electronic structure of gold, influenced by relativistic effects, allows for the formation of stable cationic intermediates and the efficient activation of substrates. The review also discusses the development of new methodologies and the potential of gold catalysts in creating novel reaction pathways. Overall, the combination of experimental and theoretical studies provides a deeper understanding of the unique properties of gold in catalysis, highlighting the importance of relativistic effects in the reactivity and selectivity of gold complexes.