As a member of the European Union, Germany welcomes other members’ citizens with their valid national identity cards (British citizens, having no national identity card, require a passport)  United states, Canadian, Australian and New Zealand visitors need to present passports and can enter the country for 90 days (each year) without applying for a visa.  Anyone wishing to stay longer should contact their local German embassy for an extension visa before they go. 

If you are arriving in Germany from another EU nation which is also a signatory of the Schengen Agreement, there is no border control. If you arrive by air from another Schengen Country, you may go through the BLUE EU gate and do not need to show identification or declare anything. Member states are: Germany, France, Belgium, Holland, Luxembourg, Andorra, Denmark, Finland, Greece, Italy, Iceland,  Monaco, Norway, Sweden, Austria, San Marino, Spain, Vatican City, Portugal, Estonia, Latvia, Lithuania, Poland, Czech Republic, Slovakia, Slovenia, Hungary and Malta

NOTE  that the UK, Ireland, Liechtenstein, Switzerland and Cyprus are Schengen signatories but have not yet joined the Agreement's "Open Borders" policy. You must go through customs if you arrive in Germany or any other Schengen country from one of these or any other nations worldwide.

Schengen Agreement Information (in German)