  We introduce polynomial sets of $(p,q)$-Appell type and give some of their
characterizations. The algebraic properties of the set of all polynomial
sequences of $(p,q)$-Appell type are studied. Next, we give a recurrence
relation and a $(p,q)$-difference equation for those polynomials. Finally, some
examples of polynomial sequences of $(p,q)$-Appell type are given,
particularly, a set of $(p,q)$-Hermite polynomials is given and their
three-term recurrence relation and a second order homogeneous
$(p,q)$-difference equation are provided.
