Soak them for a while in warm water, and you'll be able to use them, yes.
However It is equally important to know that you should reserve the liquid for its essential mushroomness, and yet also that said reserved liquid should be run through a coffee filter to remove grit.
cooking.stackexchange.com/users/218/Satanicpuppy
I guess you must be referring to dried Shiitake Mushrooms.
I agree with all the answers above, but I must make some additional steps to it.
I also believe we are talking the whole mushroom.
If it is sliced mushrooms, you will need to soak them for less time.
Yes, it's best to use cold water and it will take at least couple of hours.
I would soak them in cold water for about 15 mins Pour all the water out.
Put in fresh water and keep soaking it.
I will do this step couple of times.
Remember you may save the water to use as flavouring, but you have to clean the mushroom and make sure the bitterness is gone.
Also, I must say the imported Chinese mushroom must be handled carefully as there have been reports that they may contain dangerous chemicals.
After a couple of hours, make sure the mushrooms are soft, take them out of water, and drive them thoroughly.
You may keep the water if you wish.
The mushrooms are good for foods that are steamed or stir-fry.
Hot water
If you are in rush, then you may use hot or warm water.
You still have to go through the cleaning process, but the time will be less.
Soup
If you use mushrooms for making soup, you may not need to soak it.
I would soak them for quick 10 mins and give them a clean.
Throw them straight into water and boil them to make soup.
The flavour of the mushrooms will come out nicely.
cooking.stackexchange.com/users/2951/Foodrules 
Perhaps not the 'best' way to use dried mushrooms, but if you powder them up in your spice grinder, they make an excellent thickening agent for soups/sauces.
You won't get thickening at the level of cornstarch or arrowroot, but it will thicken things and impart a mushroomy goodness.
cooking.stackexchange.com/users/5455/Wayfaring Stranger
For some a few minutes in warm water will reconstitute them.
cooking.stackexchange.com/users/1670/dmckee --- ex-moderator kitten
Depending on the mushrooms, it may take a little longer.
You could soak them in warm/hot water and it'll be quicker, but if I have the time I generally soak them in cold water for about an hour.
I'm told it retains much more of the flavour when you do that.
cooking.stackexchange.com/users/231/talon8
I was recently given a bag of dried mushrooms and am unsure how to work with them.  What's the best way to use them?
How to work with Dried Mushrooms?
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cooking.stackexchange.com/questions/8594
cooking.stackexchange.com/users/3117/Nate
You can soak them in warm water for most recipes, or use them without soaking in soups. You can also grind them up as a thickening agent for soups and sauces. For most uses, soak the mushrooms before use. For soup, this is not necessary. You can also grind them up into a powder and use as a thickener.
Soak them in water to rehydrate them. For soup, this might not be necessary. Grind them up to make a thickener for sauces.