Guam is under the jurisdiction of the U.S. Citizenship and Immigration Services (formerly known as the Immigration and Naturalization Service).  As such, all visitors to Guam are subject to the same entry requirements as visitors to anywhere else in the United States of America.  Whether in transit or actually travelling to Guam, you will need to clear Immigration. 

The immigration processing room is quite large, and is separated into two areas.  The right side is dedicated to travellers that are NOT U.S. passport or green card holders.  All U.S. citizens and permanent residents should queue on the left side of the room.  Depending on what time your flight comes in, you may be processed in as little as two minutes, or as long as one hour.  The queues are especially long when aircraft arrive from Japan.

The immigration officer will ask you a few questions, and may take a digital copy of your fingerprints and a digital photograph of your face.  Afterward, you will be sent back to the terminal (if you are connecting on another flight), or you will take an escalator to the baggage claim area.

Whether or not you have checked baggage, you will need to clear Customs.  Guam controls its own Customs and Quarantine agency, although restrictions are similar to those in effect in the U.S. mainland.  Note the Customs and Quarantine agency frequently patrols the baggage claim area with K-9 dogs, therefore if you are concealing food or narcotics in your baggage, you are very likely to be caught.  Customs officers also conduct random bag inspections.