The first American intelligence agency was established in 1775. Once an agency for the centralized collection and analysis of intelligence information has been established, the actual collection of said information can begin. There are numerous possible sources for this information. In practice intelligence agencies attempt to gather information from a myriad of sources and then collate and analyze that information. One important general source of intelligence is referred to as Humint, human intelligence.
Human intelligence can take many forms.
Refugees from oppressive regimes often have valuable information to impart regarding those regimes. Often business contacts can be excellent sources of intelligence, sometimes totally unwittingly. During wartime much help is commonly received from disgruntled occupied populations and their resistance movements. When members of hostile forces are captured in battle it is standard procedure to interrogate them to learn the state of the enemy. These types of human intelligence gathering are usually not very exciting or spectacularly enlightening but they are the very basis of intelligence work.