In this talk we explain how to describe pure types of Spin(7) structures in terms of spinors and focus on the construction of balanced examples. An 8-dimensional Riemannian manifold admitting a Spin(7) structure determined by a 4-form Ω is spin and the structure can also be described in terms of a spinor η. Balanced Spin(7) structures are a pure class and are characterized by the equation (∗dΩ)∧Ω=0 or, equivalently, by the condition that η is harmonic, that is, Dη=0 where D is the Dirac operator. For our purposes, the description of balanced structures in terms of spinors turns out to be much simpler. Our examples are products (N×T,g+gk), where (N,g) is a k-dimensional nilmanifold endowed with a left-invariant metric, (T,gk) is an (8−k)-dimensional flat torus, and k=5,6. Under these assumptions, the presence of a left-invariant balanced Spin(7) structure on the product is equivalent to the fact that (N,g) admits a left-invariant non-zero harmonic spinor. For this reason we search left-invariant metrics on N that admit left-invariant harmonic spinors. The results of our investigation are a list of 5 and 6-dimensional nilmanifolds that verify this condition, and the description of the set of left-invariant metrics with left-invariant harmonic spinors in the particular case k=5.