This talk will cover the design and use of SENSEI, a platform for in situ visualization and analysis. In situ techniques eliminate the need to write large simulation state files or other data that prevents applications from scaling to large machines. On the one hand, the SENSEI platform allows developers of simulation applications to instrument their code with the simulation API and gain access to a wide variety of I/O, analysis, and visualization tools. On the other hand, tool developers can plug their tools into SENSEI through the analysis API, thereby allowing these tools to analyze a wide range of simulations.