This guide provides the first comprehensive summary of the land mollusk fauna (slugs and snails) for Montana. It includes a checklist of species, separate keys for slugs and snails, and individual species accounts. Species accounts include a distribution map, a photograph when available, and text on physical appearance, distribution, habitat, conservation status, remarks, and selected references. A glossary of terms used in the keys and species accounts appears after the species accounts, along with a list of references cited in the body, and three appendices; the first appendix is a guide to Conservation Status, the second a table of habitat associations for all Montana species, and the third a copy of a field survey form. Following the brief Introduction are sections discussing the composition of the Montana mollusk fauna with a comparison to adjacent regions, a brief overview of the ecological significance of land mollusks, and remarks on how to study mollusks with suggestions for conducting surveys. The extant Montana fauna includes 16 species of slugs and 62 species of land snails. Of these, 7 slugs and 3 snails are introduced species (exotics), leaving 68 species (9 slugs and 59 snails) native to Montana. Eight native slugs and 15 native land snails are Montana Species of Concern (SOC); 5 SOC snails are endemic (globally restricted) to Montana. Species richness of Montana land mollusks is higher west of the Continental Divide. Eight native slugs (all of the SOC slugs) are found only west of the Divide, the ninth native slug occurring statewide. Of the native snails, 20 occur only west of the Divide, 17 only east of the Divide, and 22 on both sides of the Divide. The documented native mollusk fauna thus includes 42 species west of the Continental Divide and 39 species east of the Divide. Of the 15 SOC snails; 8 are present only west of the Divide, 4 only east of the Divide, and 3 on both sides of the Divide.