Sometimes, a spy camera is a highly engineered, well concealed fountain pen. On other occasions, it's a pigeon with a camera strapped to it.
Former chief of disguise at the Central Intelligence Agency (CIA) Jonna Mendez, discusses why photography was her favorite method of gathering intelligence. Mendez talks about several kinds of cameras that were used in the field and why sometimes a pigeon is the best spy camera.
In a recent video from Wired (below), Mendez goes into a good degree of detail, talking about how they used different kinds of cameras for espionage. Before drones became a mainstream thing, homing pigeons were used to capture aerial photos. The quality of the results were described as being comparable to satellite photos.
And of course, the great thing about homing pigeons was that they always returned home again (and never ran out of battery at just the wrong time).
Subminiature cameras like the Minox were also a great choice for many operatives. Although these devices were readily available to the public for purchase, it's small, compact nature made them highly effective.
The most exceptional camera was the one concealed in a fountain pen, which required a certain level of training in order to produce sharp and stable images; as Mendez describes, agents were trained to act like they were the tripod. Due to the camera being concealed in a pen, it wouldn't have required a great deal of effort to take it practically anywhere without it being discovered.
Finally, there is also mention of something called a micro dot. Although this isn't specifically related to photography, it's still a remarkable degree of ingenuity. A micro dot could hold a page of text in an area which was the size of a period (full-stop). Using a Fresnel lens, agents could receive and read information without it being intercepted.
During the Cold War, photography was one of the single most important methods for gathering information and also to disprove any rumors. Mendez maintains its importance today, and that only the methods have changed.