Stable isotopes are chemical isotopes that may or may not be radioactive, but if radioactive, have half-lives too long to be measured.

Only 90 nuclides from the first 40 elements are energetically stable to any kind of decay save proton decay, in theory (see list of nuclides). An additional 164 are theoretically unstable to known types of decay, but no evidence of decay has ever been observed, for a total of 254 nuclides for which there is no evidence of radioactivity. By this definition, there are 254 known stable nuclides of the 80 elements which have one or more stable isotopes. A list of these is given at the end of this article.

Of the 80 elements with one or more stable isotopes, twenty-six have only a single stable isotope, and are thus termed monoisotopic, and the rest have more than one stable isotope. One element (tin) has ten stable isotopes, the largest number known for an element.

Different isotopes of the same element (whether stable or unstable) have nearly the same chemical characteristics and therefore behave almost identically in biology (a notable exception is the isotopes of hydrogen—see heavy water). The mass differences, due to a difference in the number of neutrons, will result in partial separation of the light isotopes from the heavy isotopes during chemical reactions and during physical processes such as diffusion and vaporization. This process is called isotope fractionation. For example, the difference in mass between the two stable isotopes of hydrogen, ¹H (1 proton, no neutron, also known as protium) and ²H (1 proton, 1 neutron, also known as deuterium) is almost 100%. Therefore, a significant fractionation will occur.