This paper presents the behavior and design concept of efficient structural steel systems based on innovative applications of knee braces. Advantages of knee-braced frames (KBF) include relatively simple connections for ease of construction and reparability after an earthquake and less obstruction as compared to conventional bracing systems. Various configurations of KBFs can be designed and detailed for different levels of strength, stiffness, and ductility. KBFs are designed so that all inelastic activities are confined to the knee braces and designated yielding elements only. Key design concepts to ensure ductile behavior of KBFs are first summarized. Finally, results from experimental and analytical studies into the behavior of KBFs are briefly presented. The results show that KBFs can provide viable alternatives to conventional structural systems.