The Chest Fly exercise can help you build both size and strength in your chest, or help tone it depending on weight used, repetitions and number of sets. It is quite a challenging exercise when done properly. The key to this exercise is using proper form by keeping your arms essentially straight through out the entire range of motion.  However, most people will bend at the elbow too much and not feel the stress in their chest or work the muscles as effectively as possible. This is a good exercise for both experienced weight trainers and beginners in the gym.