It depends - if I want the quilting to stand out (like in a plain block) I use a heavier thread and maybe a contrasting color. If I want it to blend in I will use a lighter weight thread and color match to the area I am quilting (though I have been known to use wite thread and paint the stitches to match the color if the fabric is patterned). I have also used metalic thread for emphasis. Thread weights I use are 40, 50 or 60 normally - and I also use poly instead of cotton for durability.