Cross-Module Optimization (CMO) is an effective means for improving runtime performance, by extending the scope of optimizations across source module boundaries. Two CMO approaches are Link-Time Optimization (LTO) and Lightweight Inter-Procedural Optimization (LIPO). However, each of these solutions has limitations that prevent it from being enabled by default. ThinLTO is a new approach that attempts to address these limitations, with a goal of being enabled more broadly. ThinLTO aims to be as scalable as a regular non-LTO build, enabling CMO on large applications and machines without large memory configurations, while also integrating well with distributed and incremental build systems. This is achieved through fast purely summary-based Whole-Program Analysis (WPA), the only serial step, without reading or writing the program's Intermediate Representation (IR). Instead, CMO is applied during fully parallel optimization backends. This paper describes the motivation behind ThinLTO, its overall design, and current implementation in LLVM. Results from SPEC cpu2006 benchmarks and several large real-world applications illustrate that ThinLTO can scale as well as a non-LTO build while enabling most of the CMO performed with a full LTO build.