The Center for Study of Multiple Birth 
Recognized internationally as a unique resource for research and information about the causes, effects and problems of multifetal pregnancy.
http://www.multiplebirth.com/Millennium Twins
A longitudinal sociological study of twins born in the new millennium.
http://www.millennium-twins.de
Twin Research and Genetic Epidemiology
Located at St Thomas' Hospital, London, UK.
http://www.twin-research.ac.uk/
Twin Research
Newsletter, research information, survey.
http://www.twin-research.com/
International Society for Twin Studies
Events, journal, bylaws, membership information.
http://www.ists.qimr.edu.au/
Australian Twin Registry
Includes a newsletter, registry for twins, study information and conferences.
http://www.twins.org.au/
Twin Studies
From the Institute of Behavioral Genetics at the University of Colorado. A handful of studies for twins of all ages.
http://ibgwww.colorado.edu/~agross/ibg/twinsites.htm
Twin Research
Journal of the International Society for Twin Studies. Topics covered in the journal include behavioral genetics, complex diseases, endocrinology, fetal pathology and other areas of research.
http://www.australianacademicpress.com.au/Publications/Journals/Twin_R/TResearch.htm
Twins Personality Survey
A twin survey on personality, conducted by researchers at Virginia Commonwealth University.
http://www.outofservice.com/twins/
Sleep Research Study for Twins
The University of Pennsylvania is recruiting same-sex twins living in the eastern time zone who are between the ages of 25 and 59 to participate in a sleep research study.
http://www.uphs.upenn.edu/sleepctr/clinicalresearch/twins.pdf
