Need more information. How many days total and how many in each city/area? Will your trip be all or mostly in the cities? Is this a guided tour or on your own? How will you be traveling -- plane, train, bus, car?

You'll need a light jacket and sweater, along with rain gear -- umbrella for sure, possibly light rain jacket, comfortable walking shoes. If you can do carry-on, go for it. I take too many liquids to be able to do that. Still, I can travel with one small checked bag and one small carry-on and we usually go for a month. There are excellent packers here on fodors and they're sure to respond.

Sorry -- I see you're going for 10 days. In that case, you should be able to do carry-on since you won't need a lot of liquids. For 10 days (or for a month), my packing list would be 2 pair of black micro slacks, 1 lightweight jacket to dress up slightly, 5 or 6 wrinkle proof tops, 1 cardigan sweater, 1 pair nylon travel pants, 2 pr. shoes (wear one on the plane), 1 pr. flipflops or house slippers, 1 gown, 1 lightweight robe, 3 or 4 pr. undies (wash as I go), a waterproof lightweight rain jacket, umbrella, and my lightweight leather jacket (carried on the plane). I like to wear nice jeans and my bulkiest shoes on the plane. I take one cross-shoulder purse that will hold my money, credit cards, lipstick, tissues and my flat-pack umbrella. Put your liquid cosmetics into small containers so that they fit into a quart size zipper bag, show this at security, and you're good to go.

Of course you can do 10 days with a carry-on each. Many people, myself included, can do a month or more, and have. And you're lucky you're going in May. Winter is hardest, of course, because of the need to bring heavy clothing. But May in Italy is a breeze. crckwc1 has given you a good sampling (though I've never taken or worn nylon travel pants anywhere...tights or leggings, yes). The key is to coordinate items and accessorize to turn a ho-hum daytime outfit into something wearable out for a nice evening. And remember that you're never going to see these people again and that they don't give a fig about what you're wearing.

If it's a fairly casual trip, which it sounds like it is, and you're female, I'd pack something like this:

One short-sleeved dress
One skirt
Five tops (cotton or silk): 2 long-sleeved and 3 short-sleeved
1 pair jeans (wear on the plane)
1 other pair of pants or capris, plus fairly heavy leggings if you are slim
1 sweater (cardigan or pullover, but cardigan works better with a dress)
1 waterproof, lightweight jacket
really lightweight long underwear in case you encounter unseasonably cold weather
Nightwear (I usually just pack a T-shirt for this)
Underwear
Socks
2 pairs of shoes (wear the heavier ones on the plane)

Everything else crckwc1 has said

Well I don't beleive in forcing my packing for any trip to fit into a gym bag. For 2 or 3 night busines trips I will do a carry-on.

For anyhting longer I check a moderate size bag - 24" - and if it's a longer trip - or I'm combining business and vacation I will take a 26" bag. (But remember that much of my carry-on is taken up with a computer and whatever other work things I can't ship home).

For one thing I won't wear undies washd by hand - not do I like paying almost as much to have them washed as it did to buy them. And I like having several changes of clothes - both for day time touring and going out to dinner/or concert or ballet or whatever afterward.

(Yes, we go to a nice dinner every night and do one special dinner in each city - so I do need at least 4 evening outfits for the trip as well as 6 or 7 daytime outfits. With mixing and matching that's enough for 2 weeks.)

But - it really depends on how you plan on traveling, what you plan on doing in the evenings and how much you value having one tiny bag. (To me there is not advantage - since I don't deal with my own luggage - this is why there are cab drivers and porters.)

All you have to do is navigate through one train station and you will do carry on after that! We are just at the end of a 30 day trip to Italy and France ( weather has varied from nearly 90F to 45F) and the best decision that I made for the whole trip was to do carry on only. If I had it to do over again I would bring even LESS. I brought enough undies, etc for 8 days and I did laundry at a laundromat three times, each time taking less than an hour. I could have washed things by hand as well, but didn't really have to.

I take a 22 inch rollerboard and a small shoulder bag. anything important that I might need on the flight or during the first day of the trip goes in the shoulder bag....just in case the rollerboard has to be gate checked at the last minute.

I highhly recommend carry on , so that you don't lose time waiting for baggage claim. Not sure where you are flying into, but in Rome the wait for baggage can be long! (much less worrying about lost baggage).

You definitely will want a small travel umbrella and a jacket or rain coat. I prefer dark colors to hide any stains on clothes. I like to have one dress that wiil work for nicer places, a wrap of some kind for the plane and chilly nights,a pair of jeans or two, a few shirts that can be worn more than once. You can always get by with a little less than you think.

I agree about wearing jeans on the plane and I usually wear my heaviest/biggest shoes on tye plane.