We give bilateral pointwise estimates for positive solutions u to the sublinear integral equation u = G(σu q ) + f in Ω, for 0 < q < 1, where σ ≥ 0 is a measurable function, or a Radon measure, f ≥ 0, and G is the integral operator associated with a positive kernel G on Ω × Ω. Our main results, which include the existence criteria and uniqueness of solutions, hold for quasimetric, or quasi-metrically modifiable kernels G.As a consequence, we obtain bilateral estimates, along with the existence and uniqueness, for positive solutions u, possibly unbounded, to sublinear elliptic equations involving the fractional Laplacian,where 0 < q < 1, and µ, σ ≥ 0 are measurable functions, or Radon measures, on a bounded uniform domain Ω ⊂ R n for 0 < α ≤ 2, or on the entire space R n , a ball or half-space, for 0 < α < n. Contents 1. Introduction 2 2. Kernels and potential theory 11 3. Lower bounds for supersolutions 14 4. Quasi-metric kernels 17 5. Quasi-metrically modifiable kernels 25 6. Proofs of Theorem 1.1, Theorem 1.2, and Theorem 6.1 30 References 33