This paper studies the small time behavior of the heat content for rotationally invariant α-stable processes, 0 < α ≤ 2, in domains of R d . Unlike the asymptotics for the heat trace, the behavior of the heat content differs depending on the range of α according to 0 < α < 1, α = 1 and 1 < α ≤ 2.Definition 1.1. Ω ⊂ R d , d ≥ 2 with either finite or infinite Lebesgue measure and non-empty boundary ∂Ω is said to be a uniformly C 1,1 -regular set if there are constants r, L > 0 such that for every σ ∈ ∂Ω, the set ∂Ω ∩ B r (σ) is the graph of a C 1,1 -function Λ with ||∇Λ|| ∞ ≤ L. Here and for the remainder of the paper, B r (σ) will represent the open ball about σ with radius r.We point out that according to Lemma 2.2 in [4], uniformly C 1,1 -regular bounded domains are also R-smooth boundary domains. That is, for every σ ∈ ∂Ω, there are two open balls B 1 and B 2 with radii R such that B 1 ⊂ Ω, B 2 ⊂ R d \Ω and ∂B 1 ∩ ∂B 2 = σ. Henceforth, for any Ω ⊂ R d , we set H d−1 (∂Ω) = Hausdorff measure of the boundary of Ω, if d ≥ 2, # {x ∈ R : x ∈ ∂Ω} , if d = 1.(1.7)