This collection of Web sites about twins and twin biology starts with a short feature from The Straight Dope (1), which explains the phenomenon of superfecundation. Next is a more extensive piece from Scientific American (2), offering an interesting look at the role genetics may play in the chance of having fraternal twins. The following Web site contains a brief MSNBC story on the recent findings that fraternal, or nonidentical, twins can share the same placenta (3). The Wisconsin Twin Project at the University of Wisconsin-Madison is a long-scale psychological study focused on "the genetic and environmental influences on normal development of twins in the family context." Visitors to the project's Web site (4) will find detailed information about the ongoing research as well as FAQs for general background information. On the topic of twin studies, the next Web site contains a transcript from a recent Health Report story from the Australian Broadcasting Company's Radio National. The transcript is an interview with a researcher working with the Swedish Twin Registry (the largest twin register in the world), an important resource for studying the "relative importance of genetic and environmental influences for behavioral characteristics and diseases (5). The journal Nature offers an interesting article on how the environment in the womb can have profound effects on a child's future health without altering its DNA sequence (6). The following Web site, created by Varun Tankala, offers an excellent introduction to the phenomenon of conjoined twins (7). And with the final Web site, you need not spend another restless night wondering which Star Wars character is your cosmic twin. Not as frivolous as it might seem, this Web site, created by Jeff Potter, contains a well-accepted personality test developed by a University of California-Berkeley psychologist. Users can take the test, find their Star Wars twin, and help advance the field of psychology while you're at it (8).