Book excerpt: Looking inside the wave cycle of light -- Confining strong light fields to the wave cycle -- Extreme nonlinear optics -- Measurements at the attosecond frontier -- Controlled attosecond-varying force fields -- Attosecond metrology of the first generation: AM1.0 -- Review of the birth of the new discipline -- Tracking electronic motions at the atomic scale -- Steering electronic motions at the atomic scale -- The technology platform: vacuum beamlines -- Towards the next generation: AM2.0 -- From capturing electrons to detecting diseases