Complex numbers describe the two dimensional geometry. William Rowan Hamilton (1805-1865) researched numbers which represent the three dimensional geometry. However this study was extremely difficult. After Hamilton had spent more than ten years desperately studying, in 1843, while walking beside the Royal Canal, he suddenly discovered the quaternion algebra over real numbers with a basis {1, i, j, k}. He stopped walking and carved equations on a stone nearby a bridge; i2 = j2 = k2 = ijk = -1With Hamilton's discovery, both Arthur Cayley (1821-1895) and John T. Graves (1806-1870) independently discovered the octonion numbers. Through Frobenius, Wedderburn and other mathematicians, quaternion numbers connected to the modern algebra of the 20th century. Today, quaternion numbers are applied to science, in particular, to the space industry and the CG industry. The objective of this book is to introduce and describe complex rings, quaternion rings and octonion rings. Several structuretheorems are stated from ring theoretic view points. In presenting of this book, we hope the reader will be able to feel the atmosphere of quaternion numbers.