Year 999 (CMXCIX) was a common year starting on Sunday (link will display the full calendar) of the Julian calendar. Depending on how one defines a millennium, 999 was either the last or second-to-last year of the first millennium.

In mathematics, the repeating decimal 0.999… (sometimes written with more or fewer 9s before the final ellipsis, for example as 0.9…, or in a variety of other variants such as 0., 0.(9), or ) denotes a real number that can be shown to be the number one. In other words, the symbols "0.999…" and "1" represent the same number. Proofs of this equality have been formulated with varying degrees of mathematical rigor, taking into account preferred development of the real numbers, background assumptions, historical context, and target audience.

Nine hundred [and] ninety-nine is the natural number following nine hundred and ninety-eight and preceding one thousand. 999 is a Harshad number, a Kaprekar number, a repdigit in bases 10 and 36, and a palindromic number in bases 10, 14, and 36. It is the largest 3-digit decimal integer.