PURPOSE: This workshop will describe several new open source software tools for conducting health economic evaluations. Participants will observe how to build and execute various model types using these software packages and will gain understanding of which tools may be most appropriate for their future work. DESCRIPTION: We will discuss the well-known problem of reproducibility in economic models, highlighting several critiques of traditional Excel-based modeling approaches, including: 1) missing documentation of formal model tests 2) lack of transparency of code 3) limited ability to scale and extend model structure 4) difficulty to share and verify results, and 5) absence of version control and audit trails. We will offer novel open-source software as a potential solution to these problems. The next section will introduce several new tools for building models 1) HEEMOD (Markov Models for Health Economics Evaluations) 2) DICE (Discretely Integrated Condition Events) and 3) R-core. Each has distinct advantages and disadvantages which will be discussed. We will then construct a simple Markov cohort cost-effectiveness model on a hypothetical pharmaceutical using all three software. For HEEMOD, this model will be built interactively and parts of the code will be written with audience participation. The other software based models will be run and graphics will be generated in real time. In the last section, we will show the implementation of a previously published microsimulation model. The Sick-Sicker model incorporates time- and age-dependence. We will evaluate the cost-effectiveness of a treatment that differentially impacts sick and sicker individuals. We will walk through the already constructed model, showing the code required to run the model in both DICE and R-core. The generated outputs will be shown. We will then compare the experiences building the models with each tool and compare the time to build as well as time to run.