This book describes physicists' search for a theory of the universe and the current state of their understanding, focusing on the author's own area of expertise: superstring theory. I found it rewarding, thought-provoking, and illuminating, though at times very challenging.  As a footnote in chapter ten states, "Some of the ideas in this and the next few sections are rather subtle, so don't be put off if you have trouble following every link in the explanatory chain..."<br><br>I like the way the book is organized. Before Greene goes deep into string theory, he has chapters on special relativity, general relativity, quantum mechanics, and on the difficulty reconciling the last two of these with each other. This sets the stage for string theory, a theory which enables the merging of general relativity and quantum mechanics. The middle of the book then presents the main framework of string theory, leaving the final chapters to delve into its connection to M-Theory, black holes, and cosmology. Greene describes difficult concepts without diving into mathematics -- mostly successfully, though I was left baffled on a couple of occasions -- leaving it to the chapter notes to hint at the mathematical details. (I note that I have a mathematics degree, but it doesn't encompass the math of string theory.)<br><br>I love the quest to find the ultimate underpinnings of the universe, a quest that has some of the same flavor as fundamental philosophical questions. The possibilities that the book raises are fascinating: that the fundamental ingredients of the universe may not be elementary particles, but rather vibrating strings or branes; that the universe may not have begun in a pure big bang singularity, but in a tiny Planck-size nugget of strings; that the familiar four space/time dimensions may be outnumbered by additional dimensions curled up in strange Calabi-Yau shapes. Highly recommended.<br><br>About my reviews: I try to review every book I read, including those that I don't end up enjoying. The reviews are not scholarly, but just indicate my reaction as a reader, reading being my addiction. I am miserly with 5-star reviews; 4 stars means I liked a book very much; 3 stars means I liked it; 2 stars means I didn't like it (though often the 2-star books are very popular with other readers and/or are by authors whose other work I've loved).