Monetary eligibility for unemployment insurance (UI) benefits depends on worker earnings during the base period, a twelve month interval that precedes filing the claim for benefits. While monetary eligibility requirements vary considerably from one state to the next, most use earnings during the earliest four calendar quarters of the five completed quarters immediately preceding the claim. To be deemed monetarily eligible, the worker?s earnings during the full twelve months of the base year (or base period) and during the three months of highest earnings (the high quarter) must exceed minimum thresholds as specified in the state?s UI statute. Other requirements may also be imposed such as minimum base period weeks of employment (with earnings above a specified threshold in each week), minimum base period hours worked and minimum combined earnings for the two highest quarters of the base period. In most states the claimant who does not satisfy the regular base period earnings requirements will have no possibility of collecting UI benefits. Since earnings requirements are typically expressed as minimum dollar thresholds, workers with low wage rates and intermittent labor force attachment are thought to be disproportionately excluded from eligibility. At present six states have provisions for an alternative base period in their UI laws. Workers who do not meet the regular base period monetary eligibility requirements can have their eligibility assessed under an alternative base period. This report examines the effects of alternative base period arrangements. A major focus is the numbers and characteristics of workers who qualify under the alternative base period. The analysis of Parts I and II utilizes summary information supplied by six states and tabulations of micro data from three states. Part III then considers effects on potential and actual benefit outlays and makes rough estimates of the effects on aggregate UI trust fund outlays. Issues of UI program administration occasioned by the alternative base period are examined in Part IV. The costs of applying the alternative base period to persons already eligible under the regular base period are considered in Part V. Part VI has concluding observations.