Influential parts of the veterinary profession, and notably the American Veterinary Medicine Association, are promoting the routine neutering of cats and dogs that will not be used for breeding purposes. However, this view is not universally held, even among representatives of the veterinary profession. In particular, som veterinary associations in Europe defend the view that when reproduction is not an issue, the neutering, particularly of dogs, should be decided on a case-by-case basis. However, even in Europe the American view is gaining ground. In light of this situation, this paper considers whether or not routine neutering of cats and dogs, in cases where uncontrolled reproduction is not an issue, can be ethically defended. The starting point of this consideration is a review of the veterinary literature on the effects of neutering on companion animals. The focus is both on the welfare of neutered animals themselves, and on behavioral and other effects that may not directly affect the animals' welfare, but that may be motivating factors for owners to neuter their companion animals. Here it becomes clear that justification for routine neutering, particularly of confined male dogs, does not follow from claims about the dogs' own welfare. The costs of neutering male dogs, in terms of increased risk of very serious diseases, may well outweigh the benefits. Then, building on this veterinary material, but including some other, additional considerations, the paper goes through some possible ethical approaches to routine animal neutering. These ethical approaches offer different degrees of concern about, or opposition to, routine neutering. Finally, based on this ethical exploration, it is argued that routine neutering, at least in the case of non-free-ranging companion animals, raises significant ethical questions, and from some ethical perspectives, looks highly problematic.