so I am running into an issue in my campaign where one of my players is aiming for specific areas of an enemy’s body hoping for crippling the target, I don’t exactly know how to handle that like if he says I aim for his knee cap and lands a hit I can’t exactly permently prone my monsters but I don’t wanna diminish his play experience either, any help on what I can do here would be great 

Yea I actually hadn’t considered the enemy’s aiming for specific locations back at them but thinking about it now makes perfect sense if they can do it, thanks 

There are games with ore detailed hit location rules and special effects to disabling wounds to limbs etc you could crib from. D&D isn't really designed for that out of the box.

-5 for a called shot seems good.

Definitely let the monsters do it too. I apply that to any special new things the players want to add to the game. It can also give a quick handle on how balanced it really is. If the players scream "no fair" when it is used against them they are probably looking for an unfair advantage.

I think of it this way: a loss of HP doesn't 1 to 1 translate to a wound on the body. Rather, it can represent the energy expended to dodge or parry a blow, or the shock of absorbing one with your armor, or in more extreme cases, being clipped by an arrow or nicked by a sword. That makes fights where multiple weapon hits later people are still standing make more sense, to me.

That said, a "called shot" to the knee isn't necessarily a crossbow bolt *in* the knee that might result in a fight-ending incapacitation or lasting effect, unless the enemy's HP (or fatigue, if you wanna think about it that way) is so low that they can't prevent it.

Instead, I would handle that called shot to the knee or what have you as a shove action, since you might lunge at your enemy with a sword only to have them block or something in a way that turns your focus from trying to cut off the leg, to having to try and overpower your enemy to the ground.

You could still homebrew a rule for called shots and crippling injuries, but I would say then that the enemy should have to be below a certain HP threshold for it to succeed.

I think a very simple and easy solution is let the player do that and give a description of them managing to hit the enemy's knees and how their armor absorbed most of the hit but now they limp. You don't have to give it mechanical weight but just going the extra mile of describing it would really help out.

Seems like one of those things where you, as the DM, have free licence to make the called shot as effective or ineffective as you want.  In some scenarios, it might be beneficial to the story, the party, or the encounter to let a target lose effective use of an arm.  Other times it will just get in the way, either over complicating the situation or just being irrelevant to it if it's a non-character.

Probably a case-by-case thing.

Definitely remind them that if they target, then the enemies target as well, and when they're fighting intelligent creatures, that will cripple them so much more. You have to think, if a eye is shot out, then that gives them penalty/disadvantage in charisma because it's such a grotesque injury. I normally let my players do a little extra on a critical, but other than that no targeting at my table.

I really don't like called shots because 1) it's an attempt to bypass the enemy's HP and 2) every creature is already doing their best to injure the enemy. Your character can't do any better than they already are! It's kind of like asking to ignore the enemy's armor or to score a hit without rolling.

Instead, I'd recommend using the Lingering Injury rules in the DMG so when the player randomly gets lucky and scores a crit, they can injure the enemy in some way.