One of the most common questions I get when people first start lifting is if they should do cardio. Many beginner and even intermediate lifters tend to skip cardio because they claim "you lose muscle." Well in reality this can be true but in the majority of cases cardio is only beneficial. Cardio is often used for people trying to lose weight but is also beneficial for people trying to gain weight. When I get this question I often respond with "You know those big football players? Well they do cardio every day." Cardio is great for toning and shredding your muscles but that doesn't mean it's reducing your muscle mass. For weight lifters trying to gain muscle, doing cardio three even four times a week is actually beneficial. Cardio is great for your overall health and of course your heart health. For people trying to gain mass, stick to the stair climbers, stationary bike, and treadmill. You want to do either high intensity intervals or keep your heart rate between 130-140 for optimal results. Doing so will get that you a nice sweat without putting your body into fat burning mode. If you want to run long periods of time at a jog then you're going to lose some muscle because of the calories burned and the reduction of fat cardio promotes. There's nothing wrong with doing cardio, I actually recommend it, just make sure you go into it with plan.