Log Entries:
* Captain's Log, Supplemental - A full-scale Dominion invasion appears imminent. Still, I remain confident in my crew's ability to face this crisis as they have so many others - with dedication and with courage.

Stellar Cartography
Internment Camp 371 can be found on the Gamma Quadrant map, within Dominion Territory, on page 75 of the book Star Trek: Star Charts by Geoffrey Mandel.

From "In Purgatory's Shadow" until the finalé, "What You Leave Behind", all recurring guest stars are presented in the credits with their character's name beneath their own.

Benjamin: Remember, this is a reconnaissance mission. You are to avoid Dominion ships at all costs. I want you back here in one piece.
Worf: What about Garak?
Benjamin: I want him back, too. I suppose I don't have to tell you to keep a close eye on him.
Worf: At the first sign of betrayal I will kill him, but I promise to return the body intact.
Benjamin: I assume that's a joke.
Worf: We will see.

Garak: Frankly, I think I can be quite an asset to Starfleet. With my extensive experience, I could skip the lower ranks entirely and begin my career as a Commander. Ahh, maybe you should suggest that in your letter. Tell them you'd be honored to serve under me.
Worf: Do not play games with me. (sighs) You have no desire to join Starfleet, do you?
Garak: No, I'm afraid I don't.
Worf: Then why all of this deception?
Garak: Because lying is a skill like any other and if you want to maintain a level of excellence, you have to practice constantly.
Worf: Practice on someone else.
Garak: Mr. Worf, you're no fun at all.
Worf: (ponders) Good.
Garak: (loud sigh)

Garak: I just don't see why these Runabout replicators can't provide a more varied menu. I'd like to get my hands on that fellow Earl Grey and tell him a thing or two about tea leaves.

Purgatory/Inferno: The Divine Comedy
The title of this episode, and the title of the next episode, "By Inferno's Light", that completes this two-parter within the larger Dominion story arc, are references to The Divine Comedy by 14th Century Italian writer Dante Alighieri. The work is divided into three canticas (Latin for hymns), the first two of which are Inferno and Purgatorio. In Dante's Comedy, Purgatory is the next step between Hell (Inferno) and Heaven (Paradiso) and is where those whose sins are not as grievous as to merit being in Hell are punished, with the chance that once their punishments have been given, they will earn a spot in Heaven. The realm is also seen in the cantica as a mountain that leads to heaven. Being in Purgatory's shadow places you in Hell, just as being in Inferno's light puts you in Hell.

It is evident from Doctor Bashir's uniform that he had been kidnapped somewhere before the events of "Rapture" (see that episode's first note), when the new Starfleet uniforms were put into use on the series.