In my talk I will discuss topologically twisted compactification of 6d $(1,0)$ theories on 4-manifolds with background flavor symmetry bundles. The effective 2d theory generically has $(0,1)$ supersymmetry and a residual flavor symmetry. Evaluation of its elliptic genus thus produces an invariant of the 4-manifold equipped with a principle bundle valued in the ring of (equivariant) modular forms. By further including torsion valued invariants of $(0,1)$ 2d theories, one obtains an invariant of 4-manifolds valued in (equivariant) topological modular forms (TMF). I will describe basic properties of this map and present a few simple examples. I will also mention some byproduct results on 't Hooft anomalies of 6d $(1,0)$ theories. The talk is based on a joint work with Gukov, Pei and Vafa.