To ensure the best accuracy, it is important that you are using the proper size and the headphones securely fit in the ear. Please reference FAQ: How do I find the proper fit? to make sure you are wearing them correctly. You should also make sure that the enhancers are properly installed on the ear buds so that material is fully stretched over the bud and the window for the heart rate (HR) sensor is not covered. Please reference FAQ: How do I switch enhancer sizes? for more guidance. Once you've found your ideal size and ensure they're installed correctly, make sure the HR sensor is on. When it is on, you will see a green light on the LEFT ear bud and should hear the audio prompt stating, “Heart Rate sensor on”. For best results, insert the earphone into your ear so that the green heart rate sensor on the LEFT ear bud is fully hidden inside the ear (behind your tragus). Double check that the ear bud is touching the back of the bowl of your ear for the most accurate signal - this is often times achieved by using the largest size that fits comfortably in your ear. Lastly, the ear loop should not be pushed backwards to align with arc of the ear perfectly; it is likely there will be negative space between the headphone loop and your ear. You should hear the audible alert telling you if the ear bud needs to be adjusted if it is unable to retrieve a proper reading on your heart rate. Heart Rate accuracy can vary depending on the type of exercise you are performing; it is recommended for steady state endurance-oriented activities to ensure best accuracy. While the technology is state-of-the-art, there are limitations that may cause some readings to be inaccurate under certain circumstances. These circumstances include physical characteristics, fit, and the type of activity as outlined above. The heart rate monitor data is not intended to be used for medical purposes, nor is it intended to diagnose, treat, cure or prevent any disease or condition.