A Letter Concerning Toleration is an English translation of a Latin work, the Epistola de Tolerantia, that John Locke wrote towards the end of the year 1685, while living – often in hiding – in the Dutch Republic. The Epistola was not however published until 1689, after Locke's return to England, and the English translation followed very shortly after. It soon met with a critical reply, in a pamphlet written by the Oxford chaplain Jonas Proast, which was to launch a polemical exchange in the course of which Locke wrote three further defences of his argument for toleration. Unlike the Epistola/Letter (hereafter: Letter), which is intense and compactly expressed, these defences are lengthy and often repetitive. But they comprise Locke's most fully elaborated statement of his case; they are valuable, too, because the pressure of controversy led him to clarify the priorities among his arguments.

Locke's period of exile in the Dutch Republic is very closely connected with the topic of the Letter, for it arose from political circumstances in which the questions of religious toleration, exclusion, and persecution played a large part.