# How to define a simple ICD case-control cohort in Atlas

In Atlas, a cohort definition starts by creating a **Concept Set** of medical codes of interest. The Concept Set is then used to create case and control cohorts using additional criteria defined in **Cohort Definitions**.

This example is a step-by-step guide to define a simple case and "all who are not cases" control cohort:

1. [Select FinnGen data release in Atlas](https://docs.finngen.fi/working-in-the-sandbox/which-tools-are-available/atlas/detailed-guide/how-to-define-a-cohort-in-atlas/selecting-configuration-in-atlas)
2. [Define a simple ICD Concept Set in Atlas](https://docs.finngen.fi/working-in-the-sandbox/which-tools-are-available/atlas/detailed-guide/how-to-define-a-cohort-in-atlas/how-to-define-a-simple-icd-case-control-cohort-in-atlas/define-a-simple-icd-concept-set-in-atlas)
3. [Define a simple ICD case cohort in Atlas](https://docs.finngen.fi/working-in-the-sandbox/which-tools-are-available/atlas/detailed-guide/how-to-define-a-cohort-in-atlas/how-to-define-a-simple-icd-case-control-cohort-in-atlas/define-a-simple-icd-case-cohort-in-atlas)
4. [Define a simple ICD control cohort in Atlas](https://docs.finngen.fi/working-in-the-sandbox/which-tools-are-available/atlas/detailed-guide/how-to-define-a-cohort-in-atlas/how-to-define-a-simple-icd-case-control-cohort-in-atlas/define-a-simple-icd-control-cohort-in-atlas)