A manifold is prime if it cannot be presented as a connected sum in a non-trivial way, where the trivial way is
If is a prime 3-manifold then either it is or the non-orientable bundle over , or any embedded 2-sphere in bounds a ball, i.e. is irreducible. So the theorem can be restated to say that there is a unique connected sum decomposition into irreducible 3-manifolds and 's.
The prime decomposition holds also for non-orientable 3-manifolds, but the uniqueness statement must be modified slightly: every compact, non-orientable 3-manifold is a connected sum of irreducible 3-manifolds and non-orientable bundles over . This sum is unique as long as we specify that each summand is either irreducible or a non-orientable bundle over .
The proof is based on normal surface techniques originated by Hellmuth Kneser. Existence was proven by Kneser, but the exact formulation and proof of the uniqueness was done more than 30 years later by John Milnor.