Abel's influence on modern mathematics is substantial. This is seen in many ways, but maybe clearest in the number of mathematical terms containing the adjective Abelian. In algebra, algebraic and complex geometry, analysis, the theory of differential and integral equations, and function theory there are terms like Abelian groups, Abelian varieties, Abelian integrals, Abelian functions. A number of theorems are attributed to Abel. The famous Addition Theorem of Abel, proved in his Paris Mémoire, stands out, even today, as a mathematical landmark. This book, written by some of the foremost specialists in their fields, contains important survey papers on the history of Abel and his work in several fields of mathematics. The purpose of the book is to combine a historical approach to Abel with an overview of his scientific legacy as perceived at the beginning of the 21st century.