The Virtual Cell allows the user to define parameters that can be used in the modeling of complex cell processes.
User-defined parameters can be added at the physiology level either as **global** parameters or as **reaction** parameters.
There are other predefined parameters such as species initial conditions or reaction rates for the different kinetic types that are generated as the model is being developed, for which the user has to provide values.
The parameter values can be numeric or expressions involving other allowable variables in the model. Parameter values that are expressions are referred to as **Functions** in the Virtual Cell.

**Viewing/Editing Parameters and Functions**

Parameters and functions can be viewed by clicking the 'Parameters and Functions' node in the 'Navigation' panel tree structure. This displays the model parameters in the 'View' panel on the right. There are 3 tabs:

- Parameters and Functions - that displays the parameters in the model and allows the user to define other parameters.
- Predefined Constants and Math Functions - that displays the predefined constants and math functions that are used in the Virtual Cell.
- Model Unit System - that displays the system of units that are used in the Virtual Cell.