The episode won the 1968 Hugo Award for Best Dramatic Presentation.
Harlan Elliison won the Writers Guild of America Award for Best Written Dramatic Episode for his screenplay.
Although Harlan Elliison received sole screen credit, Gene Roddenberry and D.C. Fontana did extensive rewrites (see below).
Although she does not even appear in it, this is Majel Barrett's favorite episode of the series.
This was DeForest Kelley's favorite episode of the series.
The filming of this episode began on February 3, 1967.
Harlan Ellison did not like the changes Gene Roddenberry made to his teleplay (in Ellison's version the problem was caused by a drug addicted crewman; Roddenberry felt there should not be a drug problem aboard his starship. Ellison's Guardian was a collection of statues, not a ring). Ellison asked to be credited as "Cordwainer Bird," a nom de plume he used for work he considered beneath his standard; Roddenberry refused. It started a feud between the men that lasted decades.
McCoy: Better risk a few drops of cordrazine.
Kirk: Tricky stuff. Are you sure you want to risk...?
McCoy: You were about to make a medical comment, Jim?
Kirk: Who, me, Doctor?
Kirk: Then what is it?
The Guardian: A question. Since before your sun burned hot in space and before your race was born, I have awaited a question.
Kirk: What are you?
The Guardian: I am the Guardian of Forever.
Kirk: Are you machine or being?
The Guardian: I am both and neither. I am my own beginning, my own ending.
Spock: I see no reason for answers to be couched in riddles.
The Guardian: I answer as simply as your level of understanding makes possible.
Spock: A time portal, Captain--a gateway to other times and dimensions, if I'm correct.
The Guardian: As correct as possible for you. Your science knowledge is obviously primitive.
Kirk: Annoyed, Spock?
Kirk: Strangely compelling, isn't it? To step through there, and lose oneself in another world.
Kirk: I've seen old photographs of this period. An economic upheaval had occurred.
Spock: It was called Depression, circa 1930. Quite barbaric.
Kirk: We seem to be costumed a little out of step with the time.
Spock: I'm afraid I am going to be difficult to explain in any case, Captain.
Kirk: Well, Mr. Spock, if we can't disguise you, we'll find some way of... explaining you.
Spock: That should prove interesting.
Kirk: You're a police officer. I recognize the traditional accouterments.
Spock: You were saying you'll have no trouble explaining it.
Kirk: My friend is obviously Chinese. I see you've noticed the ears. They're actually easy to explain.
Spock: Perhaps the unfortunate accident I had as a child?
Kirk: The unfortunate accident he had as a child. He caught his head in a mechanical... rice picker. But fortunately, there was an American missionary living close by who was actually a, uh... skilled plastic surgeon in civilian life.
Kirk: You were actually enjoying my predicament back there. At times, you seem quite human.
Spock: Captain, I hardly believe that insults are within your prerogative as my commanding officer.
Spock: If only I could tie this tricorder in with the ship's computers for a few moments.
Kirk: Couldn't you build some form of computer aid here?
Spock: In this zinc-plated vacuum-tubed culture?
Kirk: Yes, well, it would pose an extremely complex problem in logic, Mr. Spock. Excuse me. I sometimes expect too much of you.
Edith Keeler: A lie is a very poor way to say hello.
Edith Keeler: Now, I don't pretend to tell you how to find happiness and love, when every day is a struggle to survive. But I do insist that you do survive, because the days and the years ahead are worth living for! One day soon, man is going to be able to harness incredible energy -- maybe even the atom. Energy that could ultimately hurl men to other worlds in some sort of spaceship. And the men that reach out into space will find ways to feed the hungry millions of the world, and to cure their diseases. They'll be able to find a way to give each man hope and a common future. And those are the days worth living for.
Spock: Captain, I must have some platinum. A small block would be sufficient--5 or 6 pounds. By passing certain circuits through there to be used as a duodynetic field core...
Kirk: Mr. Spock, I've brought you some assorted vegetables, baloney and rolls for myself, and I've spent the other 9/10ths of our combined salaries for the last three days on filling this order for you. This bag doesn't contain platinum, silver, or gold, nor is it likely to in the near future.
Spock: Captain, you're asking me to work with equipment which is hardly very far ahead of stone knives and bearskins.
Edith Keeler: What... what on earth is that?
Spock: I am endeavoring, ma'am, to construct a mnemonic memory circuit using stone knives and bearskins.
Edith Keeler: Oh, and don't give me that ''questions about little old us?'' look. You know how out of place you are around here.
Spock: Interesting. Where would you estimate we belong, Miss Keeler?
Edith Keeler: You? At his side, as if you've always been there and always will. And you... you belong... in another place. I don't know where or how. I'll figure it out eventually.
Spock: I'm finished with the furnace.
Edith Keeler: "Captain." Even when he doesn't say it, he does.
Kirk: "Let me help." A hundred years or so from now, I believe, a famous novelist will write a classic using that theme. He'll recommend those three words even over "I love you."
Kirk: How are the stone knives and bearskins?
Spock: I may have found our focal point in time.
Kirk: I think you may also find you have a connection burning.
Spock: Yes. I'm overloading those lines.
McCoy: This looks like old Earth around 1920 or '25.
Edith Keeler: Would you care to try for '30?
McCoy: I am unconscious, or demented.
Edith Keeler: I have a friend that talks about Earth the same way that you do. Would you like to meet him?
McCoy: I'm a surgeon, not a psychiatrist.
Edith Keeler: Lots of people drink from the wrong bottle sometimes.
McCoy: Do you know what you just did?!
Spock: He knows, Doctor...he knows.
The Guardian: Time has resumed its shape. All is as it was before. Many such journeys are possible. Let me be your gateway.
Kirk: Let's get the hell out of here.