LCSim, standing for "Linear Collider Simulation", is a Java-based framework for physics event reconstruction and analysis. It was developed primarily for Linear Collider Detector studies and is currently used by several HEP groups and collaborations. The software has an extensive list of features.

LCSim uses the LCIO event model, the AIDA histogramming and plotting API, and also makes use of the JAS (or Java Analysis Studio) GUI which includes the Wired Event Display. LCSim also makes extensive use of the FreeHEP Java Libraries for its core functionality.