Some shakes are meal replacements -- others are supplements to your diet (like a protein shake). You should choose which one is right for you (and when to take it) based on your fitness goals. If you are trying to lean up, but want the extra protein to help build muscle, you should try to find a a protein shake that is low carb/low sugar and take it early in the day (ideally after an AM workout)....you want to try and avoid taking in too many calories at night when you're typically not burning as many calories. If you're trying to bulk up, I would drink 2-3 shakes per day -- definitely one after your workout and one later in the evening...eating real food is the best way to get the nutrition you need, but that's not always possible when you're trying to achieve certain fitness goals...and have a life...and can't cook. A good protein shake can be a convenient alternative.