I know a little bit about the subject because people are always saying, are you matching the hatch. My follow up usually begins with the brim of my hat being adjusted downward some more, so less eye contact is needed.
You see, I am just a simple man. Give me a handful of lures, something to toss them with and I will catch fish......matching the hatch or otherwise.
So to keep with the context of this topic I have Googled it and here is what I chose on the subject from Fly Fishing For Beginners.com.
Fly Fishing for Beginners - Matching the Hatch"There are a lot of factors when trying to determine the correct fly to be using. First a couple main points you should know. There is a lot of activity happening below the surface of the water. Fish are feeding on insect larvae and nymphs. Small minnows, leeches and worms are available for food. In fact, fish feed primarily under water. With that in mind, if you want to catch fish, using nymphs or streamers is your best bet. But, in my opinion, nothing compares to laying that dry fly out into the seam, waiting for that fish to rise to take the fly, and BAM, seeing the fish come out of the water to take it. Swiftly set the hook and bring it in.
In general, hatches of certain insects generally occur during specific times of the year. For instance, large salmon flies hatch in western Montana in May and June. Most local fly shops or sporting good stores will have hatch charts showing what insects are hatching for a particular time of year. You can also find hat charts at http://www.theflyfishingreport.com/.
Using hatch charts will get you close, but it helps to know the exact insect and size. A good way to determine this is to take a small net (like those used to catch fish in an aquarium) and catch anything floating on the surface of the water. You can also hold the net underwater to determine what nymphs may be floating by. Keep in mind that what you catch in the net (especially underwater) can differ from bank to mid-stream. Try your best to match what is in the net to your fly selection.
Yet another great source is your local fly shop's web site. Many fly shops update their sites daily describing what flies are working in the area. Sometimes the hardest part is determining what the fly looks like from some the obscure names that are sometimes listed on the sites.
Last, I'm a firm believer of simplicity. It's been said that many flies are now tied to catch fisherman, not fish. Keep to the basic patterns. That is all you should need."
The other thing to come across while searching was this sweet reel that is appropriately named HATCH.