Between 1965-1972 , many builders used aluminum wire instead of copper wire in their houses. While it seemed like a good cost-saving measure at the time, many of the houses had fires as a result. This wire is still found in millions of homes today, but it is far from an ideal situation.
The Problem
The fires from this wire tended to be located at junctions of outlets and receptacles, although problems could occur at the service panel. Since aluminum tends to expand and contract more than copper, it sometimes created a gap between itself and the connection, causing the electricity to arc and thus catch fire.
The Solution
There have been various methods to repair homes that have aluminum wire. Some of these fail under laboratory testing and should be avoided. Below is a list from the least involved to the most:
Do nothing. Some people are not worried about the wire since the house has not shown any problems in 50+ years. This might be fine, but the problem with the wire tends to appear over time and could show up without warning.
Check all junctions to make sure they are tight. This is time consuming, but it can help to identify potentially dangerous events before they happen. There is no guarantee that the junction will not loosen in the future, however.
Install copper wire pigtails onto the aluminum at the junctions. There are different methods used to actually do this. Some have shown to work great while others should be avoided. Even electricians can get this wrong and will use one of the unapproved repairs. I recommend reading the pamphlet by the Consumer Product Safety Commission on these methods at https://www.cpsc.gov/s3fs-public/516.pdf
Run new wire throughout the house. This is the most thorough method, but it can cost thousands of dollars.
Stranded Aluminum
One final note. The issue with aluminum wiring is with solid strand aluminum wire and not stranded. You will often find stranded aluminum (or an alloy version) used even on new homes today by the utility company or at light fixtures, etc. This wire has not been shown to be a problem.
If your house was built near 1965-1972, there is a chance it has aluminum wire and should be evaluated further by a licensed electrician. If you want a more in-depth look at the use, concerns, and methods of repair for homes with solid strand aluminum wire, I highly recommend the pamphlet by the Consumer Product Safety Commission on these methods at https://www.cpsc.gov/s3fs-public/516.pdf