Where to start
If you have never thought hard about what an IQ score is, start with how IQ tests work — it covers the g factor, the difference between fluid and crystallized ability, and why a test needs population norms before its number means anything. Then read the IQ score chart to see how a raw number translates into a percentile.
If you arrived here because of a specific question — joining Mensa, or having a child assessed — the Mensa guide and the kids guide are self-contained. For lighter reading on the numbers that get attached to famous people, and why almost none of them are real measurements, see the blog.
None of this is a substitute for taking a test. Ours is free and takes about 25 minutes; how it is built and scored is documented in the methodology, and if you would rather warm up first, the practice sets are untimed and unscored.