Placeholders in code samples below are delimited by angle brackets (i.e. `<placeholder-name>`). You will need to replace them with the values that are correct in _your_ situation for the code samples to run properly.
Unleash allows you to gather [**impression data**](../advanced/impression-data.md) from your feature toggles, giving you complete visibility into who checked what toggles and when. What you do with this data is entirely up to you, but a common use case is to send it off to an aggregation and analytics service such as [Posthog](https://posthog.com/) or [Google Analytics](https://analytics.google.com/), either just for monitoring purposes or to facilitate [A/B testing](../topics/a-b-testing.md).
This guide will take you through everything you need to do in Unleash to facilitate such a workflow. It will show you how to send data to Posthog as an example sink, but the exact same principles will apply to any other service of the same kind.
In addition, you'll need to have a toggle to record impression data for and an [Unleash client SDK](../sdks/index.md) that supports impression data. This guide will use the [JavaScript proxy SDK](../sdks/proxy-javascript.md).
Because impression data is an **opt-in feature**, the first step is to enable it for the feature you want to gather data from. You can use both the UI and the API.
![The create feature toggle form. There's a toggle at the end of the form that enables or disables impression data. It's labeled "impression data".](/img/enable-impression-data.png)
To create a feature toggle with impression data enabled, set the `impressionData` option to `true` in the request payload, as seen below. For more options, check the [reference docs on creating features](../api/admin/feature-toggles-api-v2.md#create-toggle).
<ApiRequest verb="post" payload={{name: "<feature-toggle-name>", impressionData: true}} url="api/admin/projects/<project-id>/features" title="Create a feature toggle with impression data enabled."/>
### Enabling impression data for existing feature toggles {#step-1-existing-toggles}
To enable impression data for an existing toggle, use the "edit" button on the toggle's page in the admin UI. It will take you to a form that looks like the toggle creation form. Use the "impression data" toggle to enable impression data the same way you would when [enabling impression data for a new feature toggle](#step-1-new-toggles).
![The create feature toggle form. There's a toggle at the end of the form that enables or disables impression data. It's labeled "impression data".](/img/enable-impression-data-existing-toggle.png)
Attach an event listener to Unleash that listens for `"impression"` events. Inside the listener, transform the event data to the format you want and send it to your analytics service.