Preheat oven to 400 degrees. Start the water to boil. While the water is ramping up to a fast boil, open canned tuna and drain well.
Cook Shells and Cheese per directions. I try to drain the pasta when still slightly firm, since the pasta will also be baked.
Once shells are drained, return to the cooking pan.
Add the cheese packet and drained tuna, and mix well. Put mixture into an ovenproof 2 quart casserole. I think a round deep one gives better results than a shallow casserole dish, although it works in either.
Top with crushed crackers, pop in oven and bake for 8 - 10 minutes, or until crackers have browned lightly.