When you look at the sky on a clear night, you probably enjoy the twinkling of stars. However, if you are an astronomer, you might find it extremely annoying. Almost as soon as telescopes were invented, astronomers realized that the quality of images was limited by atmospheric turbulence, which distorts short exposure and blurs long exposure astronomical images. Even if we have 8-10 meter class telescopes now, without AO, the telescope's angular resolution would be limited by the atmospheric turbulence “seeing” -the size of the blurred image is almost two orders of magnitude worse than what the telescopes could achieve. To overcome this limitation, the first concept of adaptive optics (AO) was proposed by H. W. Babcock in 1953 for ground-based telescopes. However, the technology for AO components was not mature enough then. From the late-1980s, astronomers became interested in applying the AO technique to astronomy. As of today, AO has rapidly been developed over the past 70 years and has become a state-of-the-art technique. The wavefront aberration induced by atmospheric turbulence can be measured by a wavefront sensor and compensated for by a wavefront corrector, thereby deblurring the images in part or entirely. This presentation introduces astronomical science that can be achieved with modern AO technologies. We also present recent technology trends for the AO system, such as types of wavefront sensors (WFSs) and wavefront correctors. In the wavefront sensors section, we present Shack-Hartmann WFS, pyramid WFS, and curvature WFS. In the wavefront corrector section, we introduce wavefront correctors such as deformable mirrors and a spatial liquid modulator. Lastly, we introduce modern AO systems with multiple laser and nature guide stars for various applications.