How can I loop through Items in the Item Template from an asp:Repeater?

It sounds to me like you want to use the ItemDataBound event.

http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.repeater.itemdatabound.aspx

You will want to check the ItemType of the RepeaterItem so that you don't attempt to find the checkbox in Header/Footer/Seperator/Pager/Edit

Your event would look something along the lines of:

void rptItems_ItemDataBound(object sender, RepeaterItemEventArgs e)
{
    if (e.Item.ItemType == ListItemType.Item || e.Item.ItemType == ListItemType.AlternatingItem)
    {
        var checkBox = (CheckBox) e.Item.FindControl("ckbActive");

        //Do something with your checkbox...
        checkBox.Checked = true;
    }
}

This event can be raised by adding the event in your code behind like so:

rptItems.ItemDataBound += new RepeaterItemEventHandler(rptItems_ItemDataBound);

Or by adding it to the control on the client:

onitemdatabound="rptItems_ItemDataBound"

---

Alternatively you can do as the others suggested and iterate over the RepeaterItems, however you still need to check the itemtype.

foreach (RepeaterItem item in rptItems.Items)
{
    if (item.ItemType == ListItemType.Item || item.ItemType == ListItemType.AlternatingItem)
    {
        var checkBox = (CheckBox)item.FindControl("ckbActive");

        //Do something with your checkbox...
        checkBox.Checked = true;
    }
}

You would want to do that in the Page PreRender, after the Repeater has been bound.

Try this.

for each (RepeaterItem ri in Repeater1.Items)
{
     CheckBox CheckBoxInRepeater = ri.FindControl("CheckBox1") as CheckBox;

    //do something with the checkbox
}

for (int item = 0; item < Repeater.Items.Count; item++)
{
   CheckBox box = Repeater.Items[item].FindControl("CheckBoxID") as CheckBox;
   if (box.Checked)
   {
      DoStuff();
   }
   else
   {
      DoOtherStuff();
   }
}