Without doubt, the arm muscles garner the lion’s share of interest on any physique. While I don’t condone neglecting the other body parts for one second, I do concede that for the average non-competitive bodybuilder, having an extra-good pair of arms is better than having average arms; period.
In order to build the best arms you can, you must understand the anatomy of the arm.
There arms are made up of several muscle groups, and we’ll now look at each, including some of the best exercises which develop each.
The Deltoids (Shoulders) are the round muscles which encapsulate your shoulder joint, front, side, and back. They’re purpose is to rotate and raise the arms, as well as contribute to pushing movements.
The deltoid is made of 3 heads, (Anterior, Lateral, Posterior), and we need to hit all 3 for full shoulder development.
You can do so like this:
Anterior (Front): The best exercise I know for building up the front deltoid is the military press. Seated, grab a barbell with a grip slightly wider than the shoulders. Now, keeping your back straight and still, push the barbell directly up from your chest up above your head until your arms are locked out. Now, slowly lower it into the starting position. Note: Many people find this can be painful on the shoulders, so choose to do this movement starting and finishing at eye-level; a technique recommended by Charles Glass to avoid hurting the rotor cuff.
Lateral (Side): For this, you need to do upright rows. Traditionally they would be performed with a barbell using a narrow grip, and the bar would be pulled up to the chin almost. But, since this method is known to cause rotor cuff problems, it is advised you again change tactics slightly. Use a wide grip, lean forward slightly at the hip keeping a straight back, with knees slightly bent, pull the bar up to the middle of the chest or nipple and slowly lower.
Posterior (Rear): To hit these, I use an exercise which mimics the bent over row in all mechanics, besides the fact that the bar is brought up to the upper chest/neck rather than pulled inwards towards the abdomen. With a wide grip, and your upper body bent over almost parallel to the floor and back locked out, with knees slightly bent, pull the bar up to the upper chest/neck, and slowly lower.
The Biceps are everybody’s favorite muscle, but perhaps the most 
over-trained muscle in the entire body. If it’s mass you are looking for – and it should be unless you are an advanced bodybuilder – then the simple, heavy barbell curl is what you need.
Take a grip which you are comfortable with, which will usually be around shoulder width, and keeping the elbows stationary at your sides, using the biceps curl the bar up, squeezing at the top of the contraction, before slowly lower.
(Please, be aware of the fact that the biceps make up only 1/3 of the size of the upper arm, with the triceps being the other 2/3 – and when people say they have 20” biceps in actuality they have very well developed triceps which make up the majority of the size.)
The Triceps are a big muscle located on opposite side of your bicep, and I’ve found the best way to build their size is to use close-grip bench pressing. Laying on a flat bench, clasp the barbell at around shoulder width grip, or slightly wider, and with your shoulders blades tucked solidly, lower to your upper chest, and press back up, and lower again slowly. The close grip involves the triceps to a greater degree than a wider grip.
The Forearms have two muscles which we wish to hit, there is the muscle which resides on the same side as the back of your hand, running from the wrist to the elbow, and then the more powerful underbelly of the muscle, which resides on the side of the palm.
Improving forearm strength is great because it allows you to lift heavier weights where before your grip would have failed you. Anyway, to train the forearm muscle which sits on the same side as your palm, you will perform what is known as a seated wrist curl. Simply take hold of a barbell with an underhand grip, and rest your forearms flat on your knees, keeping them steady – ensuring the barbell and your wrists hang over the edge of your knees slightly.
Now, lower the weight until it rolls down your fingers slightly, and then curl it back up.
To hit the forearm muscle that sits on the same side as the back of the hand, the movement is the same, only reverse – only the natural mechanics of this movement will prohibit the bar from being rolled down the fingers, and as this movement is far more difficult, the bar should be gripped as tightly as possible to prevent the grip from becoming limp.