Abstract. Let X be an RD-space, namely, a metric space enjoying both doubling and reverse doubling properties. In this paper, for all s ∈ [−1, 1] and p, q ∈ (0, ∞], the authors introduce the grand Besov spaces AḂ s p,q (X ) and grand Triebel-Lizorkin spaces AḞ s p,q (X ), and prove that whenfor all admissible β and γ, whereG 0 (β, γ) is the space of test functions. As applications, the authors obtain some real interpolation results on these grand Besov and Triebel-Lizorkin spaces. The corresponding results for inhomogeneous spaces are also presented.