In this paper the main focus lies on the development and the size of the shadow economy and of undeclared work (or shadow economy labor force) in OECD, developing and transition countries. Besides informal employment in the rural and non-rural sector also other measures of informal employment like the share of employees not covered by social security, own account workers or unpaid family workers are shown. The most influential factors on the shadow economy and/or shadow labor force are tax policies and state regulation, which, if they rise, increase both. Furthermore the discussion of the recent literature underlines that economic opportunities, the overall situation on the labor market, and unemployment are crucial for an understanding of the dynamics of the shadow economy and especially the shadow labor force.