How prevalent was spying during World War 1 and 2?
In WW1, there was a significant amount of spying, but it proved to be of limited importance. National spy organizations had yet to be developed, so spying was still conducted as part of the various diplomatic organizations, which meant it was not coordinated or directed.
In WW2, espionage was taken to a whole new level. Huge networks of people spied for the Allies, providing enormous amounts of information of very critical importance (and, in fact, there was even spying BETWEEN the Allies, as they tried to look at each other's secrets for political gain). Unfortunately for the Axis, the compromising of the Enigma communications system meant that Axis spying was spectacularly UNsuccessful, with virtually all Axis spies being caught, or, in fact, turning out to be double agents for the Allies.