A dog is born with its testicles inside its abdominal cavity. These must descend to the scrotum through the inguinal canal.
After the dog is 8 weeks old, we should touch them. In some cases, only one might be found or when touched, they ascend a little towards the inguinal canal, from where they came. However, if at this age we don't feel anything, it MIGHT be a case of Cryptorchid. We will allow 2 more months for the pup to develop properly before making a definite diagnostic.
Cryptorchid: it means that the testicles of a dog haven't descended; it can be unilateral or bilateral; unilateral is more common.
When the testicle is at the inguinal canal, it is easier to be put in its proper place, as long as it has a regular anatomy. In other cases, the testicle(s) might be found at the abdominal cavity. In the majority of such cases, it would need to be removed.
It is not convenient to leave these testicles inside the organism since this carries a high probability of becoming a tumour in the coming years. A surgery to remove them is always the best advice.
As soon as we realize there is a problem and before the pup is 6 months old, there is a hormonal treatment to try to help them descend without surgery. Nevertheless, it only works sometimes.
The testicles of a 2 month old puppy. As you can see, the left testicle has descended further than the right one. However, this is normal at this age.