For a pair of words in any free distributive lattice, a procedure is g
iven which decides whether they can be unified. Bases for all unifiers
are found in the case where the two words to be unified are AND-words
(or by duality OR-words). The cases where the total number of generat
ors is at most three are discussed in detail. This includes all the po
ssible combinations of constants and variables. Bases for the unifiers
are given in all these cases.