We design, develop, validate, and deploy WTF (Where's The Fault?), a system that determines whether a performance problem in a home network lies with the ISP or inside the home network. WTF uses four independent maximum like- lihood detectors to detect both access link bottlenecks and wireless network pathologies with high detection rates and low false positive rates; we use extensive controlled exper- iments to determine the appropriate thresholds for each pa- rameter that we measure. We implemented WTF as cus- tom firmware that runs in an off-the-shelf home router and deployed it in 64 home networks across 15 countries. The real-world deployment sheds light on common pathologies that occur in home networks. We find that wireless bottle- necks are significantly more common than access link bot- tlenecks, that the 5 GHz spectrum consistently outperforms the 2.4 GHz spectrum, that many homes experience high TCP round-trip latencies between wireless clients and the ac- cess point, and that performance can vary dramatically across wireless devices, even within a single home network.