What technology/knowledge could you bring to the Middle Ages?

[ignoramus]

What's needed before they can harness electricity?

[Exian]

Google glass.

[Wyrd of Gawd]

Hot air balloons and helicopters.

[ignoramus]

Dentistry! Ever noticed all medieval portraits have their mouth closed for a good reason. My friend flicker anyony? Hehe

[Ashground]

What's needed before they can harness electricity?

You could build a Voltaic pile. The pile is made of up of zinc and copper plates with vinegar- or brine-dampened pieces of leather or pasteboard placed in between each plate. The plates were then stacked in alternating order one on top of the other with the bottom plate and the top plate acting as the positive and negative terminals. This was the first serious attempt at building a battery in the 1700's. Although the Parthians (In ~250BC) arguably made the first batteries by putting a copper cylinder (With an iron rod sticking out of the top) and vinegar in a clay pot, apparently they used it to electroplate silver.

You need a means of power transmission (wires).

Also, a working knowledge of Ohm's law would be useful (Voltage= Current x Resistance/Current= Voltage ÷ Resistance/Resistance= Voltage ÷ Current). Although this works best with an ideal conductor.

Got all that? Now all you need to do is to manufacture something that will need electrical power.

I'm with Min, printing press. Yes they used it to make Bibles but the Age of Reason might never have happened otherwise.

[Homeless Nutter]

[...]
So, with the knowledge of the modern world - however much of it you currently possess, in any areas - what technology/ideas/theories could YOU introduce to the people of 1015?
[...]

Nothing. F*** the people of 1015 - I'm not going.

[robvalue]

I'd give them this device which plugs straight into the brain, and it allows you to read a book without instantly believing everything it says.

[ignoramus]

[...]
So, with the knowledge of the modern world - however much of it you currently possess, in any areas - what technology/ideas/theories could YOU introduce to the people of 1015?
[...]

Nothing. F*** the people of 1015 - I'm not going.

Ok, Ok... I'll throw in one of these but that's my final offer!

(Armour piercing of course in case a knight gives you any trouble)

[Sappho]

I'd just help the nation I like most and make them become wealthy and healthy.
I won't try to introduce complete strange ideas, because you can't change dumb people who already know close to nothing.
Then start wars and conquer the nations I don't like, without modern technology ofc cause I know too little about that.

[Anomalocaris]

They are not ready to use hygiene, much less steam engine, if these were dropped before their faces.  The Romans had both hygiene and labor saving technologies like large scale waterworks.   Men besotted by high Christianity had no use for them.

If I can bring a technology back, it would have to be a disruptive technology that they would actually strongly be tempted to use.    As with any fellowers of religions of piece, men of the middleages would enjoying destroying the works of their fellowmen second only to killing their fellow men.

So I would bring back gunpowder.   This I can guaranty they would use, and it would let the peasants topple men on horses and hasten the end of the disgusting middleages.