This talk will review the recent progress in constraining low-energy effective field theories (EFTs) using certain fundamental properties (unitarity/causality/locality) of the underlying high-energy completion. These constraints, known as ``positivity bounds'', provide valuable UV guidance for our low-energy model-building since they can diagnose when a particular set of EFT couplings is secretly unphysical (i.e. has no standard UV completion). However, the strongest such bounds to date applied only to EFTs of scalar fields. I will particularly highlight how these constraints can be developed for EFTs of massive spinning fields, presenting some new bounds which can be even stronger than their scalar counterparts.