[View or download sample code](https://github.com/aspnet/Docs/tree/master/aspnetcore/fundamentals/environments/sample) ([how to download](xref:tutorials/index#how-to-download-a-sample))
ASP.NET Core reads the environment variable `ASPNETCORE_ENVIRONMENT` at application startup and stores that value in [IHostingEnvironment.EnvironmentName](https://docs.microsoft.com/dotnet/api/microsoft.aspnetcore.hosting.ihostingenvironment.environmentname?view=aspnetcore-2.0#Microsoft_AspNetCore_Hosting_IHostingEnvironment_EnvironmentName). `ASPNETCORE_ENVIRONMENT` can be set to any value, but [three values](https://docs.microsoft.com/dotnet/api/microsoft.aspnetcore.hosting.environmentname?view=aspnetcore-2.0) are supported by the framework: [Development](https://docs.microsoft.com/dotnet/api/microsoft.aspnetcore.hosting.environmentname.development?view=aspnetcore-2.0), [Staging](https://docs.microsoft.com/dotnet/api/microsoft.aspnetcore.hosting.environmentname.staging?view=aspnetcore-2.0), and [Production](https://docs.microsoft.com/dotnet/api/microsoft.aspnetcore.hosting.environmentname.production?view=aspnetcore-2.0). If `ASPNETCORE_ENVIRONMENT` isn't set, it will default to `Production`.
* Calls [UseDeveloperExceptionPage](https://docs.microsoft.com/dotnet/api/microsoft.aspnetcore.builder.developerexceptionpageextensions.usedeveloperexceptionpage?view=aspnetcore-2.0#Microsoft_AspNetCore_Builder_DeveloperExceptionPageExtensions_UseDeveloperExceptionPage_Microsoft_AspNetCore_Builder_IApplicationBuilder_) and [UseBrowserLink](https://docs.microsoft.com/dotnet/api/microsoft.aspnetcore.builder.browserlinkextensions.usebrowserlink?view=aspnetcore-2.0#Microsoft_AspNetCore_Builder_BrowserLinkExtensions_UseBrowserLink_Microsoft_AspNetCore_Builder_IApplicationBuilder_) when `ASPNETCORE_ENVIRONMENT` is set to `Development`.
* Calls [UseExceptionHandler](https://docs.microsoft.com/dotnet/api/microsoft.aspnetcore.builder.exceptionhandlerextensions.useexceptionhandler?view=aspnetcore-2.0#Microsoft_AspNetCore_Builder_ExceptionHandlerExtensions_UseExceptionHandler_Microsoft_AspNetCore_Builder_IApplicationBuilder_) when the value of `ASPNETCORE_ENVIRONMENT` is set one of the following:
The [Environment Tag Helper ](xref:mvc/views/tag-helpers/builtin-th/environment-tag-helper) uses the value of `IHostingEnvironment.EnvironmentName` to include or exclude markup in the element:
Note: On Windows and macOS, environment variables and values are not case sensitive. Linux environment variables and values are **case sensitive** by default.
The development environment can enable features that shouldn't be exposed in production. For example, the ASP.NET Core templates enable the [developer exception page](xref:fundamentals/error-handling#the-developer-exception-page) in the development environment.
The environment for local machine development can be set in the *Properties\launchSettings.json* file of the project. Environment values set in *launchSettings.json* override values set in the system environment.
When the application is launched with `dotnet run`, the first profile with `"commandName": "Project"` will be used. The value of `commandName` specifies the web server to launch. `commandName` can be one of :
* IIS Express
* IIS
* Project (which launches Kestrel)
When an app is launched with `dotnet run`:
* *launchSettings.json* is read if available. `environmentVariables` settings in *launchSettings.json* override environment variables.
Changes made to project profiles may not take effect until the web server is restarted. Kestrel must be restarted before it will detect changes made to its environment.
> *launchSettings.json* shouldn't store secrets. The [Secret Manager tool](xref:security/app-secrets) can be used to store secrets for local development.
The production environment should be configured to maximize security, performance, and application robustness. Some common settings that a production environment might have that would differ from development include:
* Client-side resources are bundled, minified, and potentially served from a CDN.
* Diagnostic error pages disabled.
* Friendly error pages enabled.
* Production logging and monitoring enabled. For example, [Application Insights](https://azure.microsoft.com/documentation/articles/app-insights-asp-net-five/).
It's often useful to set a specific environment for testing. If the environment isn't set, it will default to `Production` which disables most debugging features.
These commands take effect only for the current window. When the window is closed, the ASPNETCORE_ENVIRONMENT setting reverts to the default setting or machine value. In order to set the value globally on Windows open the **Control Panel** > **System** > **Advanced system settings** and add or edit the `ASPNETCORE_ENVIRONMENT` value.
See the *Setting environment variables* section of the [ASP.NET Core Module configuration reference](xref:host-and-deploy/aspnet-core-module#setting-environment-variables) topic.
To set environment variables for individual apps running in isolated Application Pools (supported on IIS 10.0+), see the *AppCmd.exe command* section of the [Environment Variables \<environmentVariables>](/iis/configuration/system.applicationHost/applicationPools/add/environmentVariables/#appcmdexe) topic.
Machine level environment variables are set in the *.bashrc* or *.bash_profile* file. Edit the file using any text editor and add the following statment.
For Linux distros, use the `export` command at the command line for session based variable settings and *bash_profile* file for machine level environment settings.
When an ASP.NET Core app starts, the [Startup class](xref:fundamentals/startup) bootstraps the app. If a class `Startup{EnvironmentName}` exists, that class will be called for that `EnvironmentName`:
[Configure](https://docs.microsoft.com/dotnet/api/microsoft.aspnetcore.hosting.startupbase.configure?view=aspnetcore-2.0#Microsoft_AspNetCore_Hosting_StartupBase_Configure_Microsoft_AspNetCore_Builder_IApplicationBuilder_) and [ConfigureServices](https://docs.microsoft.com/dotnet/api/microsoft.aspnetcore.hosting.startupbase.configureservices?view=aspnetcore-2.0) support environment specific versions of the form `Configure{EnvironmentName}` and `Configure{EnvironmentName}Services`:
