According to conventional wisdom, standard lines are primarily used to provide genetic backgrounds for induction, manipulation, mapping, and mutation identification. This is the primary motivation for the development of all major zebrafish wildtype lines. As zebrafish’s use as a genetic model organism expands, new applications in fields such as toxicology and evolutionary biology will emerge. Even within the current zebrafish research community, there are frequently competing goals when it comes to the use of standard lines. A line suitable for mutagenesis, for example, may not be suitable for toxicological studies or genetic mapping. Because embryonic lethals have been removed from the population, some lines (AB, TU, and AB/TU) are frequently used for mutagenic screens. Furthermore, alleles that disrupt or interfere with embryonic phenotypes are frequently eliminated, allowing for a more productive screen for embryonic phenotypes. In some cases, specific alleles are required for complementation, enhancer, or suppressor testing. Prior to mutagenesis, appropriate genetic elements must be introgressed into the background of the standard line. Mapping strains (typically SJD and WIK) differ sufficiently from AB and TU backgrounds to yield useful sets of polymorphic molecular markers. Mapping compatibility with another line should be established in order to map new mutations quickly. SJD and WIK are the most commonly used mutation mapping genes on the AB and TU backgrounds. The highly inbred SJD provides more consistently mappable markers, but the line is more difficult to maintain for many people. WIK is also thought to be molecularly distinct enough from AB and TU to be useful in mapping. TL is a line that has not been cleaned of background mutations, but is thought to be very robust and useful for easy mutant maintenance.