More than perhaps any other outdoor activity, backpacking rewards the efficient packer and punishes the over packer.  After all, you will have to live with - and carry - every decision you make.    This list is focused on a three-season, three to five-day outing, but when packing always plan against the highest high and lowest low temperatures you'll encounter.  It'll protect you from spending a  miserably cold nigh out in the woods, give you a bit more of that oh-so-precious pack space, and save your back.    It's also good to compartmentalized when packing by putting similar items in individual bags.  Keeping all your food in one place will save you from leaving an errant energy bar behind when prepping your bear bear bag (to say nothing of preventing a bear mauling), and putting things like flashlights, matches, and your multi-tool together will help you quickly locate what you need.  Also, let things do double duty.  For example, a sleeping bag stuff sack or tent sack can make the perfect bear bag.
The Basics

• Backpack (3,00 to 5,000 cubic inches)
• Sleeping bag (rated to 20 to 40 F minimum)
• Sleeping pad
• Two-person tent/tarp

Eating and Drinking

• 2 one-liter water bottles
• Water purification (filter, iodine, or bleach)
• Stove and fuel
• Wind screen (to protect the stove flame, constructed out of aluminum foil)
• Pot/pan with lid
• Waterproof matches and lighter
• Cup or mug
• Lightweight bowl and spoon
• Multi-tool or utility knife
• Scraper for cleaning pot

Clothing

• Trail-running shoes or hiking boots (broken in and waterproofed)
• Sandals and fleece socks or lightweight camp shoes
• Wool socks
• Sock liners
• Synthetic long underwear bottom and tops
• Synthetic shorts or convertible pants
• Underwear
• Synthetic/wicking t-shirt
• Rain/wind jacket and pants
• Wool or fleece jacket (or vest if warmer)
• Wool or fleece hat*
  
• Bandanna
• Gaiters*

Accessories

• Directions, trail map, or guidebook
• Headlamp
• Toilet paper in Ziploc bag
• Plastic potty trowel
• Extra Ziploc/trash bags
• Lip balm
• Sunscreen
• Hand sanitizer
• Toothbrush and toothpaste
• First aid kit (Band-Aids/bandages, aspirin, antiseptic wipes, poison ivy treatment such as CORTAOD® Treatment Kit, moleskin, tweezers)
  
• Pack rain cover or garbage bag
• Bear-bagging cord

Optional Items

• Trekking poles
• Sun/rain hat
• Sunglasses
• Journal and pen
• Camera
• Ground cloth
• Duct tape
• Watch
• Whistle
• Small strainer (for filtering food particles while cleaning dishes)
• Contact lens wearers bring solution and back-up glasses
• *Indicates optional/depending on climate and geography