This question frequently arises on the forum, so here's a permanent page.  Do remember that it also varies a bit on where you are traveling in this great state.  The Inside passage/ Southeast tends to be cool and rainy most times of the year, but that can change and you would be surprised at the weather in the rest of the state.  You would be amazed at some tourists when they hit Fairbanks on a hot summer day - it can be 90 degrees, and they were surely not expecting that!  Add to the fact that the sun doesn't really go down, so the heat stays through out most of the evening.  But then go for a boat tour near the coast and you might find yourself freezing and staying inside sipping hot cocoa to warm up!  So let's help you get prepared for your big trip.

Okay - the real key to your packing regardless of the activities you pursue is to pack like this: LAYERS! This is really the key to traveling in Alaska during any season.  Also do remember that unless you are attending some very formal function, Alaska is very casual - clothes really need to be functional up here, and most people (no matter what the income level) really don't care much about style as well as function - really!  So it's best to stick mainly to jeans, t-shirts, a few long-sleeve shirts and/or a sweatshirt. A fleece jacket is a great thing - not heavy and easy to pack. Pack some basic summer clothes - your shorts, tank top and sandals, along with your swimsuit if you have booked a place with a pool.  A good rain coat should be included and if you are doing any hiking, some rain pants are very helpful. Your basic tennis shoes function most places, but if you are a serious hiker, you'll want your hiking boats, and perhaps some "trash" tennis shoes for river crossings.  If you are camping, a warm hat is a good addition as well. 

To make sense of the layers concept - things can change rapidly during the day, so it's best to be prepared for those changes. For example, you could be staying in Denali park and the morning is nice, so you are in jeans and a t-shirt. You hop a shuttle bus, and you arrive at Polychrome - where there is still snow, the wind is blowing and you are freezing. Easy to add a long sleeve shirt and fleece jacket - which you can remove later.  Also, there are seasons when the mosquitos are terrible and long sleeves are often your only defense, so that can help with that as well. Hope that explained layering for folks who haven't had the need in their lives.

Happy packing!