**System dynamics** (**SD**) is an approach to understanding the nonlinear behaviour of complex systems over time usingstocks, flows, internal feedback loops, and time delays.^{[1]}

The **World3** model is a system dynamics model for computer simulation of interactions between population, industrial growth, food production and limits in the ecosystems of the Earth. It was originally produced and used by a Club of Rome study that produced the model and the book *The Limits to Growth*. The principal creators of the model were Donella Meadows, Dennis Meadows, and Jørgen Randers.

The model was documented in the book *Dynamics of Growth in a Finite World*. It added new features to Jay W. Forrester‘s World2 model. Since World3 was originally created it has had minor tweaks to get to the World3/91 model used in the book *Beyond the Limits*, later improved to get the World3/2000 model distributed by the Institute for Policy and Social Science Research and finally the World3/2004 model used in the book Limits to growth: the 30 year update (synopsis).

World3 is one of several global models that have been generated throughout the world (Mesarovic/Pestel Model, Bariloche Model, MOIRA Model, SARU Model, FUGI Model) and is probably the model that generated the spark for all later models.

System dynamics is a computer-aided approach to policy analysis and design. It applies to dynamic problems arising in complex social, managerial, economic, or ecological systems — literally any dynamic systems characterized by interdependence, mutual interaction, information feedback, and circular causality.

The three most commonly used software packages are listed below in alphabetical order. Additional tools that support model construction are noted at the end.

**iThink**^{®} and **STELLA**^{®} are two names for one model development platform published by isee systems. The software is available in different configurations under a commercial license for Windows and Macintosh computers. Educational licenses and a free runtime version of the software are available.

**Powersim Studio** is available in a number of different configurations from Powersim Software. The software is available under commercial license and runs under Windows. Educational licenses and options for publishing standalone model packages are available. A new free version, Studio Express is now available.

**Vensim**^{®} is available in a number of different configurations from Ventana Systems, Inc. The software is available under a commercial license and runs on Windows and the Macintosh. Educational licenses, including a configuration of the software that is free for educational use, and a free runtime version of the software are available.

**See Also**: There are a number of other products that can be used to construct models. These include: Anylogic, Goldsim, Berkely Madonna, Sysdea and SimGua under related methodologies and MyStrategy under pedagogical tools.

OPENMODELICA is an open-source Modelica-based modeling and simulation environment intended for industrial and academic usage. Its long-term development is supported by a non-profit organization – the Open Source Modelica Consortium (OSMC).

The goal with the OpenModelica effort is to create a comprehensive Open Source Modelica modeling, compilation and simulation environment based on free software distributed in binary and source code form for research, teaching, and industrial usage. We invite researchers and students, or any interested developer to participate in the project and cooperate around OpenModelica, tools, and applications.

Simantics System Dynamics is a ready-to-use system dynamics modelling and simulation software application for understanding different organizations, markets and other complex systems and their dynamic behavior.

ASCEND is a free open-source software program for solving small to very large mathematical models. ASCEND can solve systems of non-linear equations, linear and nonlinear optimisation problems, and dynamic systems expressed in the form of differential/algebraic equations.

Insight Maker supports System Dynamics modeling: a powerful method for exploring systems on an aggregate level. By “aggregate”, it is meant that System Dynamics models look at collections of objects, not the objects themselves. For instance, if you created a model of a water leakage from a bucket, a System Dynamics model would concern itself with the quantity of water as a whole, not with individual droplets or even molecules. Similarly, if you were modeling a population of rabbits, the System Dynamics model would look at the population as a whole, not at the individual rabbits.

Sysdea modeling is based upon Stocks (something that accumulates, such as money in a bank account, trees in a forest) and Flows (the forces that cause such Stocks to accumulate and deplete). With just these two concepts and supportive Variables to allow intermediate calculations, you get great expressive power.

NetLogo is a multi-agent programmable modeling environment. It is used by tens of thousands of students, teachers and researchers worldwide. It also powers HubNetparticipatory simulations. It is authored by Uri Wilensky and developed at the CCL. You can download it free of charge.

We’ve all heard about the “Limits to Growth”. Well, the results of the computer program that started it all are published in “World Dynamics” by Jay W. Forrester (The MIT Press, Cambridge, MA, 1971; second edition, 1973). Back then you had to be at an institution to run the computer program to simulate the future world dynamics, so modeled. Nowadays you can run it on your own personal computer and play around with the model all you want (after spending a day or two going through the tutorials). All you need to do is download Vensim PLE (take care to download all files first to a known location like your “Desktop” so you can direct the installer program to them when it asks you for their location), and then open the WORLD.MDL file most likely located at: C:\Program Files\Vensim\models\sample\EXTRA\WORLD.MDL

Read on for some screen shots of the output.

PS: Meadows et al’s 2003 update to the original model is in the file WRLD3-03.VMF, most likely located at C:\Program Files\Vensim\models\sample\WRLD3-03\WRLD3-03.VMF