Early in my fly fishing years, I learned the effectiveness of emergers by their ability to catch fish before, during, and following the hatch.  Emerger patterns are those that represent insects in the stage before they are an adult, and trout have learned to key on this stage.  Many times, the insects are vulnerable during the emerger stage, thus making it a great one to imitate when fly fishing.  These include some of my favorite flies to fish during important mayfly and caddis hatches, and I recommend them to all of you.