In silico modelling of biological pathways is a major endeavour of systems biology. Here we present a methodology for construction of pathway models from the literature and other sources using a biologistfriendly graphical modelling system. The pathway notation scheme, called mEPN, is based on the principles of the process diagrams and Petri nets, and facilitates both the graphical representation of complex systems as well as dynamic simulation of their activity. The protocol is divided into four sections: 1) assembly of the pathway in the yEd software package using the mEPN scheme, 2) conversion of the pathway into a computable format, 3) pathway visualisation and in silico simulation using the BioLayout Express 3D software, 4) optimisation of model parameterisation. This method allows reconstruction of any metabolic, signalling and transcriptional pathway as a means of knowledge management, as well as supporting the systems level modelling of their dynamic activity. 2010 [2,3] and is presented here in its current form (Box 1). The scheme consists of entity nodes (which represent biochemical components), process nodes (which represent interactions between the components) and edges, which link entity and process nodes. Pathway maps can be drawn in the free graph editing software yED (yWorks, Tübingen, Germany; www.yworks.com). The bipartite (alternating entity and process node) nature of a pathway model drawn according to the rules of mEPN allows it to be parameterised for computational modelling as a Petri net (a mathematical approach for describing distributed systems, Box 2) by addition of tokens to chosen entry entity nodes in the pathway representing its initial state. When a mEPN model is imported into the open-source software BioLayout Express 3D , a simulation is performed that results in the flow of these tokens through the pathway and generation of quantitative outputs at each entity node that can be visualised as graphs of token flow over time. Altering the simulation parameters can change the flow of tokens, allowing the dynamics of pathways to be modelled under different conditions.