Spying is one of the three oldest professions: agents of the state who snoop on citizens or on the activities of other states.
But since the early 1960s, there's been an accelerating trend away from HUMINT towards ELINT (and other less-well-recognized forms of intelligence through analysis of observational data collected from non-human sources).
We're now well into the age of biometric monitoring, and this is raising huge obstacles in the face of traditional spycraft; if your spy travels on a biometric passport, then simply replacing their passport with a new one won't hide their identity from the border authorities of the nation they're visiting. Indeed, the zealous attempts of anti-terrorist security agencies to make it difficult or impossible to disguise your identity may cause extensive blowback on HUMINT operations by their governments' intelligence agencies. What does it imply about the future of espionage if a given agent can only operate in a given target nation under one identity?
Does the second oldest profession have a future in the 21st century?