Espionage is a way to weaken your enemies without actually going to war. In EU4 how espionage works will be somewhat different from EU3. In EU4 there will no longer be any spy agents, but covert actions will still be present. Rather than using spies, diplomats will be the ones performing espionage.
There's a number of new covert actions in EU4. The covert actions that have been revealed are listed below.
A diplomat can always be sent to another neighbouring nation to fabricate a claim on a neighbour province or any province you share a sea zone with.
Fabricating a claim can take up to two years, and each time your diplomat gets discovered, the progress of the claim drops by 10%. Once you have a claim on a province, you have a legal reason to start a war – a casus belli. All these claims are removed once peace is established. Claims allow you to select the targeted provice as a wargoal, and for five years after taking a claim in a peace deal you get a 50% reduction on core creation for that province.