Some powers deal half damage when they miss, and some effects, such as the weakened condition, cause damage to be halved. When a power or other effect deals half damage, first apply all modifiers to the damage, including resistances and vulnerabilities, and then divide the damage in half (rounded down).
Example: Albanon the wizard is weakened and uses ice storm, which deals 16 cold damage to a fire giant that has vulnerable 5 cold. First add 5 to the 16 cold damage for the vulnerability, giving a result of 21. Then divide that result in half for the weakened condition. The giant ends up taking 10 cold damage.
Some creatures, such as wraiths, are naturally insubstantial, and some powers can make a creature insubstantial. While a creature is insubstantial, it takes half damage from any attack that deals damage to it. Ongoing damage is also halved.
Published in Monster Manual 2, page(s) 217, Player's Handbook 3, page(s) 220, Rules Compendium, page(s) 226, Monster Vault: Threats to the Nentir Vale, page(s) 123.