Front door glass does more than let in light, it lets you see who is there before you open the door. While you can still see though cracked glass, you really should not leave it that way. Eventually, the opening and closing of the door will loosen a broken insert enough to fall out. Replace front door glass inserts is a straightforward DIY job. Most hardware and home improvement shops will cut glass to your exact measurements, which makes replacing glass inserts even easier.
Measure the existing door insert so that you will know what size to replace it with.
Remove the thin wooden frame -- called the wooden stops -- that holds the glass insert in the door, using a putty knife. Be gentle and work carefully, because you don't want to break these little wooden strips as you remove them.
Lift the broken pane from the door and discard it safely. If your door does not have wooden stops around the insert, lay an old cloth on the ground outside of the front door. Break the glass with a rubber mallet. Put on heavy gloves to pull out any remaining shards. Clean up the cloth full of broken glass.
Scrape any leftover, dried glazing compound out of the door with a corner of the putty knife.
Apply glazing compound all around the inside of the opening for the glass insert. Glazing compound comes in a "gun" very much like a caulking gun. Squeeze a thin, straight line inside of the opening where the replacement glass will go.
Place the new glass gently into place, and press it into the glazing compound. Don't push too hard, you don't want to crack the glass.
Put glazing points at each side of the glass; top, bottom, left and right. Glazing points are little pieces of metal that have a sharp edge that sticks into the wood of the door, and tiny tabs that hold the glass in place while the glazing compound dries. They are barely visible when pushed all the way in, so you can leave them in place.
Scrape away any glazing compound that might have squished out when you put the replacement glass in. A corner of the putty knife works well.
Replace the wooden stops. Carefully hammer the wire brads through the wooden stops and into the door. Wire brads are thin, straight finishing nails with a dimpled head that is meant to be sunk below the surface of the wood.
Hammer the brads beneath the surface of the wood, using the nail set -- a long piece of metal with a point at one end that fits into the dimple of the wire brad -- and the hammer.
Stay focused on the point of the nail set, and not the hammer, when using them together, to keep the nail set from slipping out of the dimple on the brad.
Never handle broken glass without thick gloves.