We describe ERIT, a collection of C routines for e ciently and reliably handling intersection queries between pairs of primitive objects in 3D. ERIT supports intersection queries between the following pairs of primitives: triangle/line-segment, triangle/triangle, sphere/linesegment, sphere/triangle, cylinder/line-segment, cylinder/triangle, cylinder/sphere, cone/linesegment, cone/triangle, toroid/line-segment, toroid/triangle, and sphere/sphere. All intersection routines are based on

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... silon-based' oating-point arithmetic. Practical tests have proved that ERIT's routines are e cient and reliable, and we provide performance statistics for three widely-used hardware platforms. The source code for ERIT is available from the author. Email: held@cosy.sbg.ac.at. Part of this work was carried out while visiting SUNY Stony Brook. 1 A toroidal solid is generated by rotating a circular arc around an axis.