Let p be an odd prime and γ (k, p n ) be the smallest positive integer s such that every integer is a sum of s kth powers (mod p n ). We establish γ (k, p n ) [k/2] + 2 and γ (k, p n ) √ k provided that k is not divisible by (p − 1)/2. Next, let t = (p − 1)/(p − 1, k), and q be any positive integer. We show that if φ(t) q then γ (k, p n ) c(q)k 1/q for some constant c(q). These results generalize results known for the case of prime moduli. Video abstract: For a video summary of this paper, please visit http://www.youtube.com/watch?v= zpHYhwL1kD0.