Germany has strict customs regulations concerning temporary importation into or export from Germany of items such as firearms, military artifacts (particularly those pertaining to the Second World War), antiques, medications/pharmaceuticals, and business equipment. Under German law it is also illegal to bring into, or take out of Germany any literature, music, or paraphernalia that glorifies fascism, the Nazi past, or the "Third Reich." Contact the German Embassy in Washington or one of the German consulates in the United States for specific information regarding customs requirements.
Surprisingly, credit cards are not accepted as widely as they are in the United States. However, automated teller machines (ATMs) are widely available throughout Germany. They utilize many of the same account networks that are found in the United States, so it is possible in most cases to get Euros directly from your U.S. bank while you are in Germany, without paying inordinate currency exchange fees.