The world would be a very different place if it were not for Albert Einstein. Like Newton and Galileo before him, this remarkable scientist changed forever mankind's understanding of the universe. In 1921, five years after proclaiming his general theory of relativity, Einstein was awarded the Nobel Prize for Physics in recognition of his remarkable achievements. In the same year he travelled to the United States to give four lectures that consolidated his theory and sought to explain its meaning to a new audience. These lectures were published the following year as The Meaning of Relativity, which he revised with each new edition until his death. It remains a key work for anyone wishing to discover at first hand the workings of one of the most inspiring minds of the twentieth century.