# Tag restrictions
<a name="tags-restrictions"></a>

The following basic restrictions apply to ACM certificate tags:
+ The maximum number of tags per ACM certificate is 50.
+ The maximum length of a tag key is 127 characters.
+ The maximum length of a tag value is 255 characters.
+ Tag keys and values are case sensitive.
+ The `aws:` prefix is reserved for AWS use; you cannot add, edit, or delete tags whose key begins with `aws:`. Tags that begin with `aws:` do not count against your tags-per-resource quota.
+ If you plan to use your tagging schema across multiple services and resources, remember that other services may have other restrictions for allowed characters. Refer to the documentation for that service.
+ ACM certificate tags are not available for use in the AWS Management Console's [Resource Groups and Tag Editor](https://aws.amazon.com/blogs/aws/resource-groups-and-tagging/).

For general information about AWS tagging conventions, see [Tagging AWS Resources](https://docs.aws.amazon.com/general/latest/gr/aws_tagging.html).