When dog puppies are born their testicles have not descended into the scrotum.  Usually by the time they are 8 weeks the testicles can be clearly felt by a vet or an experienced breeder but may take a few more weeks to descend fully.  However, occasionally, one or both do not descend but are retained inside the body; this is cryptorchidism.

Cryptorchidism is believed to be inherited, and affected dogs cannot be shown.

Surgical removal of the undescended testicle(s) is recommended as the retained testicle can become cancerous; the descended testicle should also be removed to stop unwanted breeding. Replacement with prosthetic testicles is practised in some countries but is considered unethical in the UK.