This document discusses the challenges of cross-platform attribution in online marketing. It outlines the current state of attribution, with few marketers utilizing cross-platform attribution. The two main approaches to attribution - top down and bottom up - are described. Four main obstacles to successful cross-platform attribution are discussed: viewability issues, reliance on third-party cookies, challenges with attributing mobile marketing, and difficulties integrating online and offline data. Tactics for overcoming these obstacles are suggested.