Why is it that everytime i cast the invisibility skill, enemies still know where i am? I casted it, sprinted off, went into sneak mode, and waited for the eye to close down but it always stay open. Seconds later, the enemies come and knew my exact spot. I thought its an invisibility skill, so why do enemies see me even though i am in sneak mode, ran off, hid in an isolated place and waited?

I'm bewildered too.

Two things.

1. Once they're on your trail, no matter what you do, they auto-detect you for a certain amount of time. You can see this for yourself at the giant camp near Valtheim Towers; if you can get into the cliffs above them and get their attention, you can see how long it takes them to give up even if you get out of line of sight.

2. Invisibility is a rather loud spell if you don't have Quiet Casting.

Invisibility isn't like Chameleon. Remember that! In the past, a 100% Chameleon spell will make sure enemies will never find you no matter what. Quite funny to cast it then watch them go all "Wtf? Where is s/he!"..

Invisibility always been bad in the past so not surprise with it being bad in Skyrim.

Just one thing, invisibility is a spell not a skill.

i am talking both spell and skill.
Spell = invisible spell, that i can understand since spells are loud when casted. But..
what about the vampire skill and shadow of Nocturnal? You can simply press Z and be invisible, but the enemies can always see you once they detect you. Strange

There is no invisibility skill. You are thinking of sneak. You really need quiet casting if you are going to use the invisibility spell. You can also get the effect through potions but a decent sneak is helpful either way. Maybe what you really need is some muffle enchants or lighter footwear.

i am talking about the 'Powers'. Sorry i got it messed up, but i am talking about the Powers(Vampire's invisbility power, shadowcloak of nocturnal etc). Maybe i got used to saying that they are skills but anyway.

But i got the spells sorted out. Quiet casting perk is what i needed

In Oblivion, invisibility (not chameleon) made you invisible. Nobody knew where you were and if you casted it in combat most people would stop fighting.

Morrowind and Oblivion, Invisibility was a "Get out of combat free" card. You were invisible. Now, even if you're leagues away from them, they run to your [b]exact[/i] location.

It doesn't have anything to do with Silent Casting, it's just enemies having a sixth sense about where you are even though they shouldn't even be able to see you.

If you cast it, run off, and then enter sneak mode, they'll know where you are because they heard you pounding off in the direction of your hiding spot. Sneak away instead. (You'll still need to be good enough at sneaking that they can't hear you moving away, of course.)

I don't think invisibility spell is loud. I think invisibility spell, along with all the spells in the illusion tree, are silent. But I think when you cast invisibility spell it emits a bright light, so it attracts extra attention. Being away from line of sight is helpful. A lot of detection is about noise, so muffle and/or sneaking is important when you are invisible. If I have to, I cast invisibility spell, run away from the spot a bit, then go into sneak and move away from the spot even more. I lose enemies this way quite often without running away too far.

Invisibility is not useful against Falmers because they are already blind Muffle is amazing against Falmers.

[edit] Invisibility does make noise. I was wrong about that. Thanks for pointing it out, buckom.

Casting Invisibility does make noise.

I'm pretty sure it's what others have alluded to: the enemies can still hear you.

It's all about how you use it. I was bewildered too at first how invisibility never seemed to work for me, until I realised they can still hear you. Combine it with muffle spell and then most of the time you can sneak away. The enemies will still be aware that you are somewhere, and will look for you for a certain amount of time, but will not find you unless they bump into you. Also, I'm fairy certain that having spells equipped gives out light, so enemies can still see you. Or at least, I can still see where vampires are when they turn invisible but still have spells equipped.

Apparently, drinking an invisibility potion is loud as well.
Finding it useless during combat, I made the mistake of thinking that if I ran away- you know, down a corridor and into another chamber, out of line of sight- that turning invisible would confuse my pursuers and give me a chance to regenerate a little.
No dice. They all come barreling into the room 5 seconds behind me and make a beeline for my position.
Casting candlelight would have hidden me just as well.

In my experience, casting muffle combined with even a low sneak skill makes you harder to detect than invisibility alone.

The only times I have found invisibility to be effective is when cast out of line of sight, before detection- in which case, what's the point, really?

It's hardcoded to be tied to stealth detection. I gather this was their way of phasing out the usefulness of any kind of invisibility. Basically, invisibility was reduced to a mere shader effect.

Invisibility doesn't make you inaudible, so if you sprint around you might as well not have cast it.

It works a lot better for me than what some of you are describing. My character has 100 in sneak and all the perks and when I drink the potion/cast the spell I am gone. With 'Shadow Warrior' it's almost like cheating.

I've got a sneak-mage with 70 or so in sneak. As long as I cast invisible before anyone sees me I can sneak to pickpocketing range right to their face in a lighted area. Once they've seen you, its too late.

The formula that determines whether or not an NPC will detect the player is more complicated in Skyrim than in any previous TES game.

From what I can tell these are some of the factors:

What your sneak skill is and the perks you have if you are sneaking.
Line of Sight and light sources.
There is a sight and a sound aspect to detection plus, I think at some point a random chance at detection is applied, if you are sneaking (If you have 100 sneak, all perks etc, you will never be 100% safe from detection when sneaking).
All spells make a sound as they are cast unless you have the Quiet Casting perk.
All spells cannot be seen in your hand even though you may see the spell as a light source.
Invisibility is 100% non-visible, but it does not render you silent.
Some of the factors that influence how noisy you are is your proximity to an enemy who may hear you, your speed of movement, the weight of your gear, and the Muffle effect. Swinging a weapon also makes noise dependent on the weight of the weapon. The player has a variety of ways to build noiselessness into their character, rather than one single route.

if you have the Steed Stone, or have perks to make your armor weightless, have a 100% Muffle effect or the Silence perk, and are Invisible (by spell, power or potion) you can run around without alerting anyone, so long as you don't bump into anyone or anything that may make a noise.

So basically there is no one aspect that can guarantee you are impervious to detection, which is much better than previous games. One thing I'd like changed though is to make Invisibility a percent like Chameleon in previous games, instead of an on/off 100% effect.

Not true. Invisibility isn't 100%. You can still see yourself as a blur and high level enemies will see you..I was level 98 and pretty much anybody saw me when I used Invisibility... Only way I can make it so no1 sees me is use a 100% Chameleon spell.. That actually makes you totally gone and I mean it. No blur, no you, nothing.

Just to remind folks, being invisible, doesn't make you silent

If you go storming past a bunch've bandits wearing full plate armour, backpack full of their stolen gems jangling about and your sword clattering against your greaves they ARE going to know where you are and that someone is invisible

This has been my experience as well.

Also, everything Abrinth said except for this:
I don't know how true that is. I have Sneak 100 and all the perks and wear Light Armor and I've never been detected under normal circumstances, but I wear 4 pieces of gear that have Fortify Sneak 40% on them.

Eh, I have 89 sneak and I can be crouched right up under a mob and they wont notice..

Sneaking was broken and patched at some stage, so I'm not sure just how it's working these days, but I will experiment with trying to sneak around children. There may be a way to determine the amount of randomness if I get the sneak/detect to apply 100 times.
I also have 100 Sneak with some perks and one +47% piece of enchanted gear, and I can't remember the last time I was detected. I can also run right up to a mob out of their line of sight and they have no idea I am there even without Sneak or Invis.
I think it would take controlled conditions to check if Sneak is 100% infallible though.