Secondary ion mass spectrometry (SIMS) is a potentially valuable but not fully exploited technique for problems in biological research. It is valuable because of: (1) detection of all elements and isotopes from mass 1, hydrogen; (2) high sensitivity for physiologically important elements, Na, K, Mg, Ca; (3) ion imaging of elements in areas as large as 250 micrometers in diameter with a lateral resolution of 0.5 micrometers; (4) promising efforts at quantitation at levels as low as 0.1 mmol/kg; (5) ability to analyse sequential layers to form a three-dimensional analysis. The problems which complicate its use are primarily variations in ion emission, presence of poly-atomic interferences and tissue preparation. Examples are cited of SIMS analysis of biomineralization, botany, toxicology and physiology, mostly by ion imaging techniques. SIMS has not yet been fully exploited for any single biological problem. In particular, studies using isotope discrimination, quantitative analysis and depth profiling will enhance the usefulness of SIMS as a technique for biological research.