"Informal" work refers to temporary or occasional side jobs from which earnings are presumably not reported in full to the Internal Revenue Service and which typically do not constitute a dominant or complete source of income. Perhaps the most important reason for undertaking informal work is to offset negative income and employment shocks, such as reduced hours in a formal job, stagnant wages, or involuntary unemployment. Such negative shocks affected many Americans during the Great Recession, so it is important to determine the extent to which people engaged in informal work during this sustained downturn and how much it helped them economically. New web-based applications (such as Uber) that facilitate a variety of informal earnings opportunities mean that more people may undertake informal work. The authors designed the Boston Fed's Survey of Informal Work Participation (SIWP) to identify the types of individuals who undertake in such activities, how much they earn, their current employment status, how they rate the value of informal work in helping to offset negative shocks, and to explore how the Internet and mobile applications have facilitated the opportunities for engaging in informal paid work via the "peer-to-peer" economy.