Abstract: A sight glass apparatus for viewing the interior of a pressurised vessel, chamber or a pipe conveying fluid under pressure is described. The sight glass apparatus comprises a sight glass assembly mounted over an opening into the vessel, chamber or pipe, the sight glass assembly comprising a sight glass adjacent the opening which provides a window to the inside of the vessel, chamber or pipe, and a containment vessel mounted behind and/or around the sight glass assembly for containing fluid exiting the opening in the pipe in the event that the sight glass assembly fails. In this way, even if the sight glass assembly fails, the pressurised fluid within the pressurised vessel, chamber or pipe is safely contained.