In spite of the substantial increase in information on billfish over the last decade, especially in the landing and catch per unit effort data, statistics for some of the commercial and recreational fisheries, basic biological information on billfish such as on growth, maturation, catch at age, and sex ratio by size, is still insufficient for the application of more data-demanding models. A family of production models is still the major tool for the stock assessment of billfish. Although there are significant increases in information on catch per unit effort, it is still hard to obtain solutions from these models in many cases without putting subjective constrains on the model parameters. In addition to the uncertainty in catch information, major uncertainty in the results of stock assessments comes from the difficulties in the realistic estimation of effective fishing effort for each fishery harvesting billfish. Those difficulties arise mainly from operational changes in the fishing and partial coverage of geographical distribution of billfish by the fisheries. It is very hard to overcome these problems only by improvement of the fishery-dependent statistics. Additional information on habitat preferences of billfish and the vertical distribution of longline gears, facilitated by the newly developed equipment such as pop-up archival tags and small time-depth recorders, is very useful to evaluate gear efficiency and stock availability to fishing gear. The assessment of billfish will be improved by the habitat-based standardization models utilizing these additional data..