As use of smartphone-based tools & services broadens and deepens, such products should be continuously evaluated and optimized to meet users' needs. In-product surveys are one way to gather attitudinal and user experience data at scale, in context of actual experiences. However, given space constraints, OS variance, product design differences, and app vs mobile web options, launching contextual surveys on smartphones requires considerable attention to several user experience aspects that can also impact data quality. In this talk, we will discuss a variety of practical UX considerations for in-context surveys on smartphones, drawing on real-world implementation and experimentation for several of Google's most-used mobile products. In particular, we'll discuss issues such as when to trigger a survey, sampling across platforms (mobile vs desktop), invitations vs inline questions, survey length, question types, device size, screen orientation, survey interaction with the host product/app. We will also explore the effect of design and text variants on smartphone survey response rates and response distributions.