Hi my Grohe kitchen sink faucet is loose. How to I tighten it? Is there something that needs to be turned by hand underneath to tighten or do I need special tools? Making things worse is the bottom of the sink prevents a good view of the bottom of the faucet. Below the countertop. Any ideas?

Needs to be tightened from under the sink, may need a right angle faucet wrench, they are about $10, or you can try and snug it up by hand, hope this helps?

Probably a loose fixture nut under the counter behind the sink.
Yeah ya gotta pull out everything and crawl up into the cabinet on your back and see if the nut (right up at the granite, above the hot and cold inlet pipes) is loose. If it is THEN you'll need to tighten it. Wrench, slip-joint pliers or a basin wrench. (probably basin wrench which is a specialized tool made for reaching up into tight places)

yes there is, with a pair of slip joints you can tighten from underneath, it is above the supply line, the best thing to do is turn off the water, disconnect the supply line and tighten the nut, then reconnect the supply line and turn back on the water. Good Luck Tim

http://www.needplumbingsupplies.com/Grohe-Kitchen-Sink-Faucets.asp?id=13758

This link has a spec sheet that will give you an view of under neath counter top. There should be a nut that you can tighten it up with.

You'll have to go under the sink and look up were the faucet comes through. You'll see it's held in place by a large nut. This nut needs to be tightened. Unfortunately working in the tight space under the sink is a pain in the ****, just be patient and you should be able to tighten it. TIP: Have someone hold the faucet on top in the position you want it.

there are mounting plates underneath the counter which fasten the faucet to the counter.This unit will need a small allen wrench if memory serves me correctly...keep in mind that all moen faucets are lifetime guranteed and alot of folks do not take advantage of this amazing warranty which makes these the most desirable faucet on the market.however the need to tighten this faucet prob isnt a warranty replacement procedure but can be determined once mounting items and faucet is inspected...good luck

I hope this helps.
Step11Turn off the water supply. You'll find a shut-off valve under your kitchen sink. Once the water is turned off, open up the kitchen faucets to allow the water remaining in the pipes to drain way. Step2Catch the drips by placing a bucket under the hot water supply line, located on the left side of your sink. Even a minimum of water can cause a mess and be potentially damaging. Step3Use a wrench to remove the compression nut that secures the hot water supply line. Let the disconnected section fall into the bucket. Use the wrench to loosen and remove the nut at the other end of the line and remove it as well. Step4Repeat Steps 2 and 3 for the cold water supply line. Step5Replace the cold water supply line. Hand-tighten the nuts on both ends, then use the wrench to tighten the nuts completely. Repeat the process with the hot water supply line. Step6Turn on the water supply and test for leaks

There should be an o-ring on the mating surfaces of the hose and the faucet that my be missing.  It could also me that the big washer in side of the faucet (under the cartridge) has been shifted.  
I doubt it is the cartridge itself, always try the cheap ideas first.

Unfortunately you cannot tighten the faucet from the top. you will need a 1/2" extended socket (the type for car repairs) and you will need to tighten it from underneath using an extension bar.