A temporary import license is required, See below. Equipment should be registered with U.S. Customs before shipping or travel. Equipment For Professional Use Form 4455. It's also a good idea to include copies of the original invoice of the equipment to prove you bought it in the U.S. when bringing it back to the U.S. Just extra protection.

Latest advice to musicians from German Customs:

http://www.artsjournal.com/slippeddisc/2013/01/latest-advice-to-musicians-from-german-customs-even-more-confusing.html

US CUSTOMS:

https://help.cbp.gov/app/answers/detail/a_id/368/~/registering-equipment,-computer,-camera,-laptops,-etc.-prior-to-traveling

Personal Use Form 4457

http://www.atacarnet.com/cbp-form-4457

Equipment For Professional Use Form 4455

http://www.atacarnet.com/cbp-form-4455

http://www.nicholsonprints.com/Articles/form4457.htm

EU CUSTOMS

http://www.revenue.ie/en/customs/leaflets/pn1008.html

http://en.wikipedia.org/wiki/ATA_Carnet

This might have already been covered by the employer or patron: don't forget about work permits! Many artists are turned away at the border because they told an immigration officer they're doing a job (without a visa or work permit). Sometimes it's easier to be just a tourist!