ACT-UP is a toolbox implementation of the ACT-R cognitive architecture, aimed at allowing rapid prototyping of complex models. With ACT-UP, we propose Accountable Modeling, where the only model components that are specified are those supported by empirical evidence and part of the model’s theoretical claims. ACT-UP is a library providing a programmatic interface to the classical ACT-R functionality. Implemented in a functional programming paradigm, models are reusable in other contexts. The toolbox is demonstrated using five implemented and evaluated cognitive models. Keywords: Complex Models, Cognitive Architectures,ACT-R