Although you can get away without having an overflow in your garden pond, for piece of mind we would recommend putting one in.
If there is a heavy down fall or continuous rain fall your pond will fill up with water, this water has to go somewhere.
If you have an overflow fitted the excess water will be directed away from your pond, usually to a drain to stop your pond from overflowing.
A lot depends on the location of your pond and how much rain fall you get, and to some degree how your pond is constructed.
If the pond you have, or are going to have is level with the ground around it we would advise you put in an overflow.
A problem you might encounter with a ground level pond is water from the surrounding area during a rain storm flowing into your pond, add that to the rain water failing onto the pond and it would soon overflow, so having the edge of the pond a couple inches higher will protect it from run off from the surrounding ground.
Putting an overflow into this type of pond can be a problem, because you need gravity to allow the excess water to drain from the pond.
One way around this is to dig a slopping trench for the overflow pipe to run away from the pond and into a soak away full of gravel.
If you have a pond on the top of a slope in your garden the overflow pipe can use gravity to drain away excess water, the same goes for an above ground pond.
One problem you might encounter if you have a pond using a pond liner without an overflow is what we call liner lift, this is when the water in your pond overflows over the edge of the liner and behind it lifting the liner as it fills with water.
There are lots of ways to build an overflow into your pond using pipework, just remember to put some sort of mesh or screen at the point where the pipe enters your pond to stop your fish or fish eggs from disappearing down the pipe.
If you have a skimmer on the side of your pond you can fix an overflow pipe into this, the skimmer has all the protection for your fish built in.
Your overflow should be fitted just above the natural water level, as the water rises the overflow will kick in, the water will not spill out over the sides of your pond but safely through the overflow.