I know I have said this before about other good books but I really do wonder how I missed this one. I've read a good majority of Asimov's works and considering the esteem this is held in I'm slightly lessened by not having read this earlier. I've always loved Asimov best out of the science fiction giants because of just how much science he puts into his fiction. The book doesn't disappoint in that area. The other thing I love is that even most laymen can understand the science he puts into his books. I will admit that it's a little harder to here but that's what makes this book. Once you conquer it you are able to enjoy the story as it unfolds a little more.You need a firm grounding in something when it not only switches to the “Para” universe but even more so when he begins to also explain how these aliens mate and relate. That's a pretty hard thing to do without making this book seamy ala Heinlein. Asimov not only manages but he makes you see their plight in a sympathetic light even though it's also a threat to humanity.I wouldn't recommend this as an introduction to science fiction but it's a must read for those that love the genre.