Keywords: 
• 轨道序 /
• 金属-绝缘体转变 /
• 超导电性 /
• 过渡金属氧化物 /
• 关联电子体系 /
• 电-声耦合

Abstract: Orbital degrees of freedom bring about rich and colorful quantum phases in cor-related electron systems of condensed matter, and lead to complexity, hence challenging our under-standing of the nature of strongly correlated systems. In this review the various properties of multi-orbital correlated electron systems are introduced:we first briefly describe the important role of the orbital degrees of freedom and the scenario of orbital order;next we discuss the inter-rela-tionship among orbital polarization, orbital ordering and lattice distortion in some classical orbit-al-ordered compounds, and outline the theories and experiments on detecting orbital order;then we introduce the magnetic modulation of multi-orbital metal-insulator transitions and multi-orbital su-perconductivity; finally we discuss the present problems and challenges in the research of orbital physics.