The history of the Groenhagen families in Ostfriesland, Germany, and the United States. Groenhagen Family History covers Harm Siebelts Groenhagen's (Groenhagen's second great-grandfather) descendants to the present and his ancestors back to Geert Garrels (the family didn't use the surname Groenhagen until about 1811), who was born about 1640. The book also discusses Geert Garrels' descendants in Illinois, Kansas, Minnesota, South Dakota, and Germany. Groenhagen's book also covers Harm Tammen Groenhagen's descendants to the present and his ancestors back to Garrelt Nonnen, who was born before 1685. Garrelt Nonnen's descendants can be found in Illinois, Iowa, Kansas, and Germany. Groenhagen Family History includes a chapter on genetic genealogy. Using DNA testing, Groenhagen is finally able to answer the question regarding whether or not he is related to Harm Tammen Groenhagen and his descendants.