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# Start a simulation in Postman Simulator

Using the Collection Runner or the Postman CLI, you can start all mocks in the simulation at once, or start them individually after you've applied scenarios. You need to start the mock before you can send requests to it.

To start a simulation from Postman Simulator, do the following:

1. Open a workspace and select **Tools > ![Stroke icon](https://assets.postman.com/postman-docs/aether-icons/v12/icon-simulator-stroke.svg#icon) Simulations** in the footer.

2. Select the simulation you'd like to start.

3. (Optional) Update the mocks and scenarios in the simulation. You can also turn off the toggle next to a mock to exclude it from the simulation.

4. Click **Start** to start the simulation.

5. Send requests to the mock URLs and observe the responses with the simulated conditions applied. For each mock, note the slug and include it in the request URL or header depending on how you set up the simulation.

   For example, if the simulation URL is `https://my-simulation.sim.my-team.postman.dev` and you have a mock with the slug `new-mock-049rk`, you can send a request to `https://my-simulation.sim.my-team.postman.dev/new-mock-049rk/items` or include the header `x-mock-slug: new-mock-049rk` with the request.

6. [View simulation logs](/docs/design-apis/simulate-conditions/view-edit-or-update-a-simulation#view-simulation-logs) to analyze how the simulated conditions are affecting your API's behavior.

To start a simulation with the Postman CLI:

* Use the `postman simulation run` command to start a simulation. See [Simulation commands](/docs/postman-cli/postman-cli-simulator#postman-simulation-run) for details.
* Use the `--simulate` option with the `postman collection run` command to run a collection with a simulation. See [Collection commands](/docs/postman-cli/postman-cli-collections#postman-collection-run) for details.

To change a simulation, see [View, edit, or update a simulation](/docs/design-apis/simulate-conditions/view-edit-or-update-a-simulation).