While they are en route to Starbase 515 in TNG: "Samaritan Snare", Picard is talking of his early days in Starfleet and the incident with the Nausicaans, when Wesley poses a surprising question: "Was that before the Klingons joined the Federation?" Picard confirms that. So are the Klingons members of the Federation at the time of the episode (2365) and had already been for some time? A view of the bridge of a Klingon ship from TNG: "Heart of Glory" seems to corroborate just that. Here we see the Federation emblem along with the Klingon emblem. It is not something electronically inserted into the image, but the two logos are obviously openly displayed on the Klingon vessel. Why would the Klingons do that if they were not members of the Federation? In TNG: "Hide and Q" Q casually states that humanity defeated the Klingons, providing an explanation why the Klingons might be Federation members by the time of TNG.