This is a must have fly in your box.  The Elk Hair Caddis is a classic fly that will catch trout.  Because elk hair is hollow, it floats very well in the water and makes the fly easy to see.  Make sure to dance this fly on the water by moving your rod tip.  Trout get extremely excited for this fly and the take is violent and fun to watch.  Make sure to have a variety of sizes as trout can be very particular on certain rivers and lakes.