These pages describe how to use 3-way and 4-way switches to control lights from two or more locations. This is the first of several related pages explaining how to control lights with multiple switches. Unless you already have a good understanding of the intricacies of 3- and 4-way switches, you should read at least the first three of these pages in order.
Disclaimer: I have no experience with X10 automation. I only offer this section after getting lots of questions from people who find themselves in a situation that is difficult to handle with conventional passive switches. I have no recommendations or endorsements to offer for particular brands.
Oops! You installed the wiring, covered it all with dry-wall, and have now discovered that you did not pull 3-conductor cable everywhere you should have. Is there any hope short of tearing out dry wall? Yes: X10 home automation products. You can install one standard switch that can also be remotely radio controlled from a second switch anywhere in your home. See, for example, this page on Three Way Switch Emulation with X10 products. You may also do a web search for "X10 home automation."
X10 automation appears to be a good alternative when retrofitting a home for second or third switches to control a light or lights. X10 is more expensive than standard switches, but demands less wiring. Basically, you replace the primary switch, add a control module, and add a remote unwired switch. With minimal programming, your system should work with nothing but 2-conductor cable.
Other pages by Rick Matthews: