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Knight v Knight (1840) is a landmark case in trust law in which the requirements (still followed) for determining whether a trust has been validly constituted were first promulgated. The test allowed the court to determine whether certain assets could be disposed of in the testator's will, or, in the alternative, whether the wording of the will was too vague to allow the intended beneficiaries to collect what appeared on its face to be theirs. Knight v Knight is still followed in most common law jurisdictions.

Lord Langdale MR formulated the test, known as 'the three certainties'. It specifies that for a trust to be valid the following items must be readily determinable: (i) the intention of the settlor or testator to create the trust; (ii) the subject matter (or property) of the trust; and (iii) the objects (or potential and actual beneficiaries) of the trust.

For further discussion see The 'three certainties'.