Bluefin tunas are well known for their long-distance migrations. Early tagging studies using simple ‘floy’ or ‘spaghetti’ tags first indicated that bluefin tuna could cross entire ocean basins (Clemens and Flittner 1969; Bayliff et al. 1991; Bayliff 1994). While these studies provided valuable life history information, the dates and locations of tag deployment and recovery were the only migratory data provided by these simple tags. The recent advent of electronic tagging technology has transformed our understanding of bluefin tuna migrations (Carey 1983; Gunn et al. 1994; Lutcavage et al. 1999; Lutcavage et al. 2000; Stevens et al. 2000; Block et al. 2001; Block 2005; Wilson et al. 2005; Kitagawa et al. 2007; Teo et al. 2007; Willis and Hobday 2007; Patterson et al. 2008; Kitagawa et al. 2009; Boustany et al. 2010; Fujioka et al. 2010). The data provided by electronic tags have allowed researchers to better understand migration dynamics such as trans-Pacific migrations in Pacific bluefin tuna and the extent of mixing of multiple stocks of Atlantic bluefin tuna. They have allowed for analysis of diving behavior, residency times, and even feeding success using surgically-implanted archival tags.