While it won't stop a wireless network from working altogether, interference in its frequency range can slow it down significantly, as well as reducing its range. If something is causing interference, the first thing you'll know about it is when your connection stops working -- unless you know what to look for. There are two very common causes of wireless interference: wireless phones and microwave ovens. 2.4Ghz, the most common wireless networking frequency, is also a commonly-used wireless phone frequency. It is possible, though, to find phones that use other frequencies. Microwave ovens, on the other hand, operate at around 2.4Ghz by definition. It should be alright to have devices like these in your house, but certainly not in the same room as any computer that you plan to use a wireless connection with.

Wireless can, in theory, pass through walls and other partitions easily. In practice, though, some walls are more solid than others, which means that they are more likely to block some of the signal. Note that it's only your interior partitions that matter, not the exterior ones. This does, however, include your floors, if you want the connection to work between levels. Wireless does well with partitions made from: drywall, plywood, other wood (including doors), glass. Wireless has trouble with: brick, plaster, cement, metal, stone, double-glazed glass. Basically, it's all to do with how porous the materials are -- ones that let more of other things through also let more of your wireless signal through.