Synonyms and related words :
attend, attend to, auscultate, be all ears, bend an ear, bug, bugging, cloak-and-dagger work, cock the ears, counterespionage, counterintelligence, eavesdrop, electronic surveillance, espial, espionage, examine by ear, following, give attention, give audience to, give ear, hark, hear, hear out, hearken, heed, intelligence, intelligence work, intercept, lend an ear, listen, listen at, listen in, listen to, military intelligence, observation, secret police, secret service, shadowing, sit in on, spying, stakeout, surveillance, tailing, tap, trailing, wiretapping