Set the ValidateRequest  attribute of the @Page directive to false. To do it
at the application level, set the same attribute to false in the web.config
file.

...
<system.web>
   <pages validateRequest="false" />
</system.web>
...

Just make sure you properly HTML-encode/decode the textbox content in the
code-behind.

I have a text box and I want to let the users insert HTML Tags in the tags.
It does not work and breaks if there are any HTML Tags inside the text.
Is there any way I can enable HTML tags to be part of the input text?

Thanks,

"It does not work"  the characters are stored? they are badly stored?  You
get an error?

Assuming you get an error, check out:
http://weblogs.asp.net/pwilson/archive/2003/05/05/6480.aspx

ASP.Net 1.1 has a ValidateRequest field, turned on by default, which won't
allow potentially dangerous user input...simply turn it off.

Karl

Thank u both