A computer-implemented method for providing a watermarked version of a data stream, wherein the data stream comprises a plurality of content elements, wherein a first content element of the data stream has been processed to form two or more differently watermarked copies of the first content element, wherein the watermarked version of the data stream comprises at least one of the two or more differently watermarked copies of the first content element and wherein said processing of the first content element causes a degradation in output quality of the watermarked version of the data stream following the at least one of the differently watermarked copies of the first content element in a rendering order of the data stream, the method comprising: processing a second content element of the data stream to form two or more differently watermarked copies of the second content element, wherein: in the rendering order of the data stream, the second content element is at an interval equal to or greater than a watermark interval from the first content element; in the rendering order of the data stream, none of the content elements within the watermark interval have been processed so as to form two or more differently watermarked copies of that content element; the watermark interval comprises one or more content elements in the rendering order of the content elements of the data stream and the watermark interval is set so that said processing of the first content element causes a degradation of at most a predetermined level in output quality of the watermarked version of the data stream at the end of the watermark interval following the at least one of the differently watermarked copies of the first content element in the watermarked version, wherein the watermark interval is set based on measurements of the degradation in the output quality; and the watermarked version of the data stream further comprises at least the content elements within the watermark interval and at least one of the two or more differently watermarked copies of the second content element. processing a second content element of the data stream to form two or more differently watermarked copies of the second content element, wherein: in the rendering order of the data stream, the second content element is at an interval equal to or greater than a watermark interval from the first content element; in the rendering order of the data stream, none of the content elements within the watermark interval have been processed so as to form two or more differently watermarked copies of that content element; the watermark interval comprises one or more content elements in the rendering order of the content elements of the data stream and the watermark interval is set so that said processing of the first content element causes a degradation of at most a predetermined level in output quality of the watermarked version of the data stream at the end of the watermark interval following the at least one of the differently watermarked copies of the first content element in the watermarked version, wherein the watermark interval is set based on measurements of the degradation in the output quality; and the watermarked version of the data stream further comprises at least the content elements within the watermark interval and at least one of the two or more differently watermarked copies of the second content element. in the rendering order of the data stream, the second content element is at an interval equal to or greater than a watermark interval from the first content element; in the rendering order of the data stream, none of the content elements within the watermark interval have been processed so as to form two or more differently watermarked copies of that content element; the watermark interval comprises one or more content elements in the rendering order of the content elements of the data stream and the watermark interval is set so that said processing of the first content element causes a degradation of at most a predetermined level in output quality of the watermarked version of the data stream at the end of the watermark interval following the at least one of the differently watermarked copies of the first content element in the watermarked version, wherein the watermark interval is set based on measurements of the degradation in the output quality; and the watermarked version of the data stream further comprises at least the content elements within the watermark interval and at least one of the two or more differently watermarked copies of the second content element.