This paper presents a study on asymmetric metasurfaces with high-Q resonances governed by bound states in the continuum (BICs). The research reveals that metasurfaces composed of asymmetric meta-atoms with broken in-plane symmetry can support sharp high-Q resonances, which are linked to BICs. The authors demonstrate that these resonances can be understood through the physics of BICs and their connection to Fano resonances. They develop a general theory for such metasurfaces, enabling the engineering of resonances for applications in nanophotonics and meta-optics. Metasurfaces, which are two-dimensional arrays of meta-atoms, have attracted significant attention due to their ability to control wavefronts and focus light. The study shows that various metasurface designs, including those with asymmetric structures, can be unified under the concept of BICs. These BICs are mathematical objects with infinite Q factors and zero resonance width, typically existing in ideal lossless structures. However, in practice, BICs can be approximated as quasi-BICs with finite Q factors and resonance widths. The paper demonstrates that high-Q resonances in various metasurface designs, such as those with tilted silicon-bar pairs, can be explained by BICs. The authors derive a universal formula for the Q factor as a function of the asymmetry parameter, showing that the Q factor depends quadratically on the asymmetry. This relationship allows for the smart engineering of resonances in metasurfaces. The study also shows that the Q factor of quasi-BICs can be controlled by adjusting the asymmetry parameter. The authors validate their findings through analytical and numerical simulations, confirming the relationship between the Q factor and the asymmetry parameter. The results suggest that the concept of BICs can be applied to a wide range of metasurface designs and other photonic structures, providing a new approach for the engineering of resonances in nanophotonics and meta-optics. The findings have implications for various applications, including optical sensors, nanolasers, and ultrafast active metadevices.This paper presents a study on asymmetric metasurfaces with high-Q resonances governed by bound states in the continuum (BICs). The research reveals that metasurfaces composed of asymmetric meta-atoms with broken in-plane symmetry can support sharp high-Q resonances, which are linked to BICs. The authors demonstrate that these resonances can be understood through the physics of BICs and their connection to Fano resonances. They develop a general theory for such metasurfaces, enabling the engineering of resonances for applications in nanophotonics and meta-optics. Metasurfaces, which are two-dimensional arrays of meta-atoms, have attracted significant attention due to their ability to control wavefronts and focus light. The study shows that various metasurface designs, including those with asymmetric structures, can be unified under the concept of BICs. These BICs are mathematical objects with infinite Q factors and zero resonance width, typically existing in ideal lossless structures. However, in practice, BICs can be approximated as quasi-BICs with finite Q factors and resonance widths. The paper demonstrates that high-Q resonances in various metasurface designs, such as those with tilted silicon-bar pairs, can be explained by BICs. The authors derive a universal formula for the Q factor as a function of the asymmetry parameter, showing that the Q factor depends quadratically on the asymmetry. This relationship allows for the smart engineering of resonances in metasurfaces. The study also shows that the Q factor of quasi-BICs can be controlled by adjusting the asymmetry parameter. The authors validate their findings through analytical and numerical simulations, confirming the relationship between the Q factor and the asymmetry parameter. The results suggest that the concept of BICs can be applied to a wide range of metasurface designs and other photonic structures, providing a new approach for the engineering of resonances in nanophotonics and meta-optics. The findings have implications for various applications, including optical sensors, nanolasers, and ultrafast active metadevices.