The show's over-arching themes:

'''1. Trust'''

Reflected in the series' ArcWords:
->''In the end, we're all alone and no one is coming to save you.''
Spoken by Reese to Jessica in the airport flashback in "Mission Creep" (where it also doubles as {{Foreshadowing}} of Jessica's fate); various references to "going it alone" or "you're all alone out here" appear in several episodes (''e.g.,'' the Iraq flashback in "Get Carter").
->''You have to trust somebody.''
The counterpoint to the above, first used by Finch in "Ghosts," trying to convince Theresa not to run away again; seems to show up at least once an episode. Culminates in the discussion of trust between Reese, Carter, and Fusco in "Firewall."
->'''Fusco:''' ''You can't trust us, how we supposed to trust you?''
->'''Reese:''' ''Trust is complicated, Lionel. For example, I'm sitting in a police car with one cop who tried to murder me and another who spent six months trying to lock me up. So you'll forgive me if I take things one step at a time.''
There's another counterpoint which is repeated frequently (''e.g.,'' Reese to Carter in "Get Carter," to Maxine in "Bury the Lede"):
->''You're not alone.''
This becomes extremely important in "Endgame."

'''2. Loss'''

Established in Reese's voiceover, the first lines in the pilot episode:
->''When you find that one person who connects you to the world, you become someone different. Someone better. When that person is taken from you, what do you become then?''
All four of the main characters have lost someone close to them: Reese lost Jessica (the voiceover is what he says to her killer when he confronts him in "Many Happy Returns"), Finch lost Grace and Nathan, Carter's marriage failed when her husband came back from the Middle East with PTSD (revealed in "Endgame"), Fusco is divorced (under circumstances yet to be fully explained). It's true of many of the [=POIs=] as well: Elias' entire life has been one complex exercise in avenging the loss of his mother, and Root's StartOfDarkness was triggered by the loss of her best friend as a child; Dr. Tillman in "Cura Te Ipsum" and Darren in "Wolf and Cub" are seeking revenge against someone who took a loved one away from them.

'''3. HeWhoFightsMonsters'''

As noted on the main page, Reese is a ''very'' self-aware version of this trope:
->''I know what happens when you take a life. You lose a part of yourself.''
The theme is also present in The Machine. It's "a system of unimaginable power" built to fight monsters, but it seems to have turned the people in the government who control it a little monstrous themselves. Also, many episodes question the morality and utility of vigilante action, particularly "Cura Te Ipsum"...
->''Maybe it's up to me to do what the good people can't.''
..."Get Carter,"...
->''You can't keep playing God.''
...and "Many Happy Returns."
->''There are things you can do Detective, and things you can't. And that's where I come in.''

'''4. Second Chances and Redemption'''

->''I believe everybody deserves a second chance''
Finch offers Reese a second chance at life; Reese and Finch's interventions give many of the [=POIs=] a second chance; Fusco tries to take advantage of the opportunity for one which Reese unintentionally extends him. In Fusco's case, his continuing association with Reese and Carter leads to his redemption, as he explains to Simmons in "The Devil's Share":
->''I could've been just like you, a bottom-feeder who turns on his own kind. For what? Money, power? I got lucky. I had a partner. She was good for me. For a lot of reasons. She reminded me that I could be good again too. I could be a good father, a good friend, a good cop. I'm not gonna let you undo all the good she did. Carter saved my life. She-- she saved me from myself because she believed in me. And I'm not gonna throw all that away on a piece of crap like you.''

'''5. Past Mistakes'''

Many of the characters are dealing with the consequences of, or atoning for, their past actions: Reese and Finch, of course; Fusco trying to escape his DirtyCop past; the [=POIs=] in "Mission Creep," "Triggerman," and "The High Road," to name a few. Summed up by Finch in "2-Pi-R":
->''Your mistakes, like mine, are part of who you are now. You can't move on from that. Believe me. I've made a sizable number. But... sometimes your mistakes can surprise you. My biggest mistake, for instance... brought me here. At exactly this moment when you might need some help. ''
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