Gold (Au), a transition metal with an atomic number of 79 in the periodic table of elements, was discovered in approximately 3000 B.C. Due to the ultrahigh chemical stability and brilliant golden color, Au had long been thought to be a most inert material and was widely utilized in art, jewelry, and finance. However, it has been found that Au becomes exceptionally active as a catalyst when its size shrinks to the nanometer scale. With continuous efforts toward the exploration of catalytic applications over the past decades, Au nanomaterials show critical importance in many catalytic processes. Besides catalysis, Au nanomaterials also possess other promising applications in plasmonics, sensing, biology and medicine, due to their unique localized surface plasmon resonance, intriguing biocompatibility, and superior stability. Unfortunately, the practical applications of Au nanomaterials could be limited because of the scarce reserves and high price of Au. Therefore, it is quite essential to further explore novel physicochemical properties and functions of Au nanomaterials so as to enhance their performance in different types of applications.Recently, phase engineering of nanomaterials (PEN), which involves the rearrangement of atoms in the unit cell, has emerged as a fantastic and effective strategy to adjust the intrinsic physicochemical properties of nanomaterials. In this Account, we give an overview of the recent progress on crystal phase control of Au nanomaterials using wet-chemical synthesis. Starting from a brief introduction of the research background, we first describe the development history of wet-chemical synthesis of Au nanomaterials and especially emphasize the key research findings. Subsequently, we introduce the typical Au nanomaterials with untraditional crystal phases and heterophases that have been observed, such as 2H, 4H, body-centered phases, and crystal-phase heterostructures. Importantly, crystal phase control of Au nanomaterials by wet-chemical synthesis is systematically described. After that, we highlight the importance of crystal phase control in Au nanomaterials by demonstrating the remarkable effect of crystal phases on their physicochemical properties (e.g., electronic and optical properties) and potential applications (e.g., catalysis). Finally, after a concise summary of recent advances in this emerging research field, some personal perspectives are provided on the challenges, opportunities, and research directions in the future.