In simple terms, a milestone is a significant event that is important for the entire project. In comparison to tasks, project management milestones usually do not have any duration, as they are just landmarks to measure the progress in-between project completion.
For example, it can be report finalization, budget approval, receipt of external review, or completion of a certain phase.
In GanttPRO Gantt chart maker software, there are several ways to add milestones. Thus, you can easily add them regardless of whether you work on a chart, with cards, or a list of tasks.
Thus, you can add them:
- Through the list of tasks on a project grid
- Through advanced buttons in the list of tasks on a project grid
- Through the Gantt chart
- By converting a task into a milestone
Milestones are visualized on a chart as a diamond-shaped checkpoint.
Through the list of tasks on a project grid
To add a milestone to a parent task, click on the three dots to the right of the corresponding task and select Add a child milestone.
You can also click on the three dots to the right of any task under the parent task and select Add a sibling milestone. In this case, a milestone will be added at the same level of that task.
To add a milestone to a particular task from the group of the parent task, click on the three dots to the right of the corresponding task and select Add a child milestone.
Through advanced buttons in the list of tasks on a project grid
To use this option, you have to open the Views settings 1, enable Show advanced buttons 2, and then click on Add a milestone 3 below the tasks in a corresponding parent task.
This milestone will be a child milestone to a parent task and a sibling milestone to the subtasks.
Through the Gantt chart
To add a milestone straight from the Gantt chart, right-click on a corresponding task bar and select Add a sibling milestone if you want the same-level milestone, or Add a child milestone if you want to add a milestone for this particular task. 
By converting a task into a milestone
You can convert a task into a milestone by clicking on the three dots to the right of the corresponding task and then on Convert into milestone 1, or right-clicking on the corresponding task bar on a Gantt chart 2.
Another option is to change task type in Task settings.
It is also possible to establish dependencies between milestones and between milestones and tasks. For this, use any of the methods mentioned here for creating dependencies between tasks.