In mathematical logic, a **conservative extension** is a supertheory of a theory which is often convenient for proving theorems, but proves no new theorems about the language of the original theory. Similarly, a **non-conservative extension** is a supertheory which is not conservative, and can prove more theorems than the original.

More formally stated, a theory is a (proof theoretic) conservative extension of a theory if every theorem of is a theorem of , and any theorem of in the language of is already a theorem of .

More generally, if is a set of formulas in the common language of and , then is **-conservative** over if every formula from provable in is also provable in .

Note that a conservative extension of a consistent theory is consistent. If it were not, then by the principle of explosion, every formula in the language of would be a theorem of , so every formula in the language of would be a theorem of , so would not be consistent. Hence, conservative extensions do not bear the risk of introducing new inconsistencies. This can also be seen as a methodology for writing and structuring large theories: start with a theory, , that is known (or assumed) to be consistent, and successively build conservative extensions , , ... of it.

Recently, conservative extensions have been used for defining a notion of module for ontologies: if an ontology is formalized as a logical theory, a subtheory is a module if the whole ontology is a conservative extension of the subtheory.

An extension which is not conservative may be called a **proper extension**.

- ACA
_{0}, a subsystem of second-order arithmetic studied in reverse mathematics, is a conservative extension of first-order Peano arithmetic. - Von Neumann–Bernays–Gödel set theory (NBG) is a conservative extension of Zermelo–Fraenkel set theory with the axiom of choice (ZFC).
- Internal set theory is a conservative extension of Zermelo–Fraenkel set theory with the axiom of choice (ZFC).
- Extensions by definitions are conservative.
- Extensions by unconstrained predicate or function symbols are conservative.
- IΣ
_{1}(a subsystem of Peano arithmetic with induction only for Σ^{0}_{1}-formulas) is a Π^{0}_{2}-conservative extension of the primitive recursive arithmetic (PRA).^{[1]} - ZFC is a Σ
^{1}_{3}-conservative extension of ZF by Shoenfield's absoluteness theorem. - ZFC with the continuum hypothesis is a Π
^{2}_{1}-conservative extension of ZFC.^{[citation needed]}

With model-theoretic means, a stronger notion is obtained: an extension of a theory is **model-theoretically conservative** if and every model of can be expanded to a model of . Each model-theoretic conservative extension also is a (proof-theoretic) conservative extension in the above sense.^{[2]} The model theoretic notion has the advantage over the proof theoretic one that it does not depend so much on the language at hand; on the other hand, it is usually harder to establish model theoretic conservativity.