This book, originally published in 1921, is written as an introduction of the theory of relativity of Albert Einstein. Moszkowski wrote the book in a way simple to understand for everyone. He developed the content in close personal discussions and it offers wonderfull view into the human Albert Einstein and his life and work.The writer Alexander Moszkowski lived from 1851 to 1934. He was born in Pilica (Poland), lived most of his life in Germany. Moszkowski was well known with Albert Einstein and probably the first author to open Einstein\'s theory of relativity to a wide audience.This book is a reprint of the original published in 1921.