Although wildland fires triggered by lightning are a natural, inevitable, and in many cases a necessary ecological process, past federal fire suppression policies have led to an accumulation of fuels and contributed to larger and more severe wildland fires. In recent years, both the number of acres burned by wildland fires and the costs to suppress fires have been increasing. From 1995 through 1999, wildland fires burned an average of 4.1 million acres each year; from 2000 through 2004, the fires burned an average of 6.1 million acres each year—an increase of almost 50 percent. During the same periods, the costs incurred by federal firefighting entities to suppress wildland fires more than doubled, from an average of $500 million annually to about $1.3 billion annually. Although efforts to fight these larger, more severe fires have accounted for much of the increase in suppression costs, the continuing development of homes and communities in areas at risk from wildland fires and the efforts to protect these structures also contribute to the increasing costs. Forest Service and university researchers estimate that about 44 million homes in the lower 48 states are located in the wildland-urban interface. When fire threatens the wildland-urban interface, firefighting entities often need to use substantial resources—including firefighters, fire engines, and aircraft to drop retardant—to fight the fire and protect homes. As wildland fire suppression costs have continued to rise, increasing attention has focused on how suppression costs for multijurisdictional fires are shared. To share suppression costs for a specific fire, local representatives of federal and nonfederal firefighting entities responsible for protecting lands and resources affected by the fire—guided by the terms of the master agreement—decide which costs will be shared and for what period. They document their decisions in a cost-sharing agreement for that fire. According to federal officials, cooperating entities traditionally shared suppression costs on the basis of the proportion of acres burned in each entity’s protection area because the method was relatively easy to apply and works well when the lands affected by a wildland fire are similar. Officials said that the use of alternative cost- sharing methods has been increasing in recent years. Federal and nonfederal entities included in our review used a variety of methods to share the costs of fighting fires that burned or threatened both federal and nonfederal lands and resources. Although master agreements between federal and nonfederal entities typically listed several cost- sharing methods, the agreements often lacked clear guidance for officials to follow in deciding which cost-sharing method to apply to a specific fire. Consequently, for eight fires we reviewed in four states, we found varied cost-sharing methods used and an inconsistent application of these methods within and among states, although the fires had similar characteristics. The type of cost-sharing method chosen is important because it can have significant financial consequences for the federal and nonfederal entities involved. Master agreements provide the framework for federal and nonfederal entities to work together and share the costs of fighting wildland fires. The master agreements we reviewed for 12 western states all directed federal and nonfederal entities to develop a separate agreement, documenting how costs were to be shared for each fire that burned—or, in some cases, threatened to burn—across multiple jurisdictions. The master agreements varied in the cost-sharing methods specified: The master agreement for 1 state (Idaho) did not identify any specific cost- sharing method to use. The master agreements for 3 states (Alaska, Arizona, New Mexico) listed the acres-burned method as the primary or only method to be used. Although two of these agreements allowed the use of alternative cost- sharing methods, they did not explicitly state under what circumstances an alternative method would be appropriate. The master agreements for 8 remaining states listed multiple, alternative cost-sharing methods but did not provide clear guidance on when each method should be used. Federal and nonfederal entities used varied cost-sharing methods for the eight fires we reviewed, although the fires had similar characteristics. As shown in figure 1, the cost-sharing methods used sometimes varied within a state or from state to state. The costs for the two fires that we reviewed in Utah were shared using two different methods, although both fires had similar characteristics. For the Blue Springs Fire, federal and nonfederal officials agreed that aircraft and engine costs of protecting an area in the wildland-urban interface during a 2-day period would be assigned to the state and the remaining costs would be shared on the basis of acres burned. Federal and state officials explained that, because the Blue Springs Fire qualified for assistance from the Federal Emergency Management Agency (FEMA), state officials agreed to bear a larger portion of the total fire suppression costs. For the Sunrise Complex of fires, in contrast, state officials were reluctant to share costs in the same manner. Although these fires also threatened the wildland-urban interface, they did not meet the eligibility requirements for FEMA reimbursement of nonfederal costs. Consequently, federal and nonfederal officials agreed to share costs for the Sunrise Complex on the basis of acres burned. The costs for the two fires we reviewed in Arizona were also treated differently from each other. For the Cave Creek Complex of fires, federal and state officials agreed to share suppression costs using an acres-burned method for the southern portion of the complex, which encompassed federal, state, and city lands and required substantial efforts to protect the wildland-urban interface. The federal government paid the full costs for the northern portion of the fire. For the Florida Fire, federal and nonfederal officials were unable to reach an agreement on how to share costs. Officials from the affected national forest proposed a cost-sharing agreement, whereby the state would pay the costs of firefighting personnel, equipment, and aircraft used to protect the wildland-urban interface, and all other fire suppression costs would be paid by the federal government. The state official, however, did not agree with this proposal. He believed that the Forest Service, not the state, was responsible for protecting areas of the wildland-urban interface threatened by the Florida Fire and that he was not authorized to agree to the terms of the proposed agreement. Methods used to share suppression costs for fires with similar characteristics also varied among states. For example, costs for the fires we reviewed in California and Colorado were shared using methods different from those used for similar fires we reviewed in Arizona and Utah. In California, federal and nonfederal officials agreed to share the costs of two fires using the cost-apportionment method—that is, costs were apportioned on the basis of where firefighting personnel and equipment were deployed. Officials said that they had often used this method since the mid-1980s because they believed that the benefit it provides in more equitable cost sharing among affected firefighting entities outweighs the additional time required to apportion the costs. In Colorado, federal and nonfederal officials agreed to share suppression costs for both of the fires we reviewed in that state using guidance they had developed and officially adopted in 2005, called “fire cost share principles.” Under these principles, aviation costs for fires burning in the wildland-urban interface are shared equally for 72 hours, and other fire suppression costs, such as firefighting personnel and equipment, are shared on the basis of acres burned. Having clear guidance as to when particular cost-sharing methods should be used is important because the type of method ultimately agreed upon for any particular fire can have significant financial consequences for the firefighting entities involved. To illustrate the effect of the method chosen, we compared the distribution of federal and nonfederal costs for the five fires we reviewed in which the actual cost-sharing method used was not acres burned with what the distribution would have been if the method used had been acres burned. We found that the distribution of costs between federal and nonfederal entities differed, sometimes substantially, depending on the cost-sharing method used. The largest differences occurred in California, which used the cost apportionment method. For the Deep Fire, using the cost-apportionment method, federal entities paid $6.2 million, and nonfederal entities paid $2.2 million. Had the costs been shared on the basis of acres burned, federal entities would have paid an additional $1.7 million, and nonfederal entities would have paid that much less because most of the acres burned were on federal land. According to federal and state officials, the nonfederal entities bore a larger share of the cost than they would have under an acres-burned method because of the efforts to protect nonfederal lands and resources. For the Pine Fire, using cost apportionment, federal entities paid $5.2 million, and nonfederal entities paid $8.1 million. Had an acres-burned method been used, federal entities would have paid about $2 million less, and nonfederal entities would have paid that much more. According to a federal official who worked on apportioning costs for that fire, the higher costs that the federal entities paid under cost apportionment were largely due to extensive firefighting efforts on federal land to ensure that the fire was extinguished. In Colorado and Utah, the differences in federal and state entities’ shares between the methods used and the acres-burned method were less pronounced, likely because the cost-sharing methods used still relied heavily on acres burned. In each case, federal entities’ shares would have been more and nonfederal shares less had an acres-burned method been used, due to the efforts to protect the wildland-urban interface. For example, the federal share of costs for the Blue Springs Fire in Utah would have been about $400,000 more and the nonfederal share that much less if an acres-burned method had been used for the whole fire. In Colorado, we estimated that the federal share of costs for the Mason Gulch Fire would have been about $200,000 more and the nonfederal share that much less under an acres-burned method. Federal and nonfederal agency officials we interviewed raised a number of concerns about the current cost-sharing framework.