Transparent textures?

I want to apply a texture with transparent sections (i.e.: alpha values) on a polygon, without seeing the polygon where the texture is transparent (like the trees on Mario64, or the monsters in Doom). However, when I apply the texture on the poly, I see the poly''s own color where the texture is transparent. I tried to set the poly''s alpha to totally transparent, but now I can''t see the poly (which is nice) but I can''t see the texture either (which is not). How can it be done ? Eric Laberge