In a method of recognising an optically encoded binary word, the word is chosen from a subset of binary words for which WOT=0 if W=T, WOT=1 if W¸T is true, where T is the target word, W is the word to be compared and O is the operation formula (I). The word is recognised by carrying out in the optical domain a bit-wise Boolean AND operation. The operation is carried out on the word to be recognised and the complement of the target word. The AND operation may be carried out by introducing the word to be recognised and the target word into a nonlinear crystal and detecting the second harmonic output from the crystal. The complement of the target word may be generated by a fixed word-forming network. The method may be applied to recognising the address of a packet on an optical network.