Star Trek Season 3 Episode 2

Kirk: The design of the ship is the same. Mr. Spock? You said you had a theory on why your sensors didn't pick up the new ships until they were upon us.
Spock: I believe the Romulans have developed a cloaking device which renders our tracking sensors useless.
Kirk: If so, the Romulans could attack into Federation territory before we would know they were there. Before a vessel or a planet could even begin to get their defenses up.
Scott: They caught us right enough.
Kirk: Well that's a brilliant observation, Mr. Scott. Do you have any other helpful opinions?