In 1999, the annual "WOW Worship" series was launched. Rather than using the year, each release is identified by the color of the cover. The worship series and hits series are today among the best-selling members of the product brand. Beginning in 2008, WOW began releasing separate single-disc collections spread throughout each year, rather than a two-disc collection toward the end of each year. In addition to "WOW Hits", which focuses on the most popular pop and pop-rock music, plans call for either single-disc "WOW Rock Hits" and "WOW Pop Hits" collections released twice a year.