The corporate income tax gap (CIT Gap) is the gap between corporate tax revenues as they "should be" collected and as they "are" collected. The gap is an indication of potential CIT revenue losses. The topic has gained in prominence in the public domain given its impact on public finances, on the level playing field between companies and on the overall tax morale. Estimating the CIT gap is therefore very relevant. It is however also very complex. This report aims at mapping different methodologies and approaches for estimating the CIT gaps and explaining their advantages and disadvantages. The report does not provide an exhaustive review of the economic literature and statistical techniques for deriving at these estimates but it provides an overview of a number of methodologies used in Member States or other jurisdictions, devised by international institutions, or presented in the literature. This report defines the CIT gap as encompassing both non-deliberate actions by taxpayers (such as errors or omissions) and deliberate actions (such as fraud, evasion and avoidance) that lead to shortfall in revenues. This report reflects the objective of the Tax Gap Project Group (TGPG) to map and share expertise and good practices. The two main approaches to estimating the tax gap - the top-down and bottom-up methods - have both advantages and disadvantages. The choice of the estimation method depends heavily on the availability of data, resources and purposes of the estimate. While the top-down methods start from macroeconomic indicators or national accounts data to estimate the CIT gap, bottom-up methods start from data obtained from individual taxpayers and extrapolate them to a wider population. There are clear complementarities between both approaches. From the findings of the report, it seems too early to identify a consensus methodology, which could be used across countries and provide for overall tax gap estimations. By providing an overview of the state-of-the-art and highlighting the strengths and weaknesses of each method, the report is nevertheless a first step to in that direction. However, the large differences in CIT systems point to the main difficulty of the exercise, which is to agree on one or more benchmarks. This makes international comparisons difficult because they depend to a large extend on the choice of the benchmark. The report also stresses that the focus should be on the trend of the results rather than on the absolute values. Currently, about ten Member States have taken steps or already estimate a CIT gap with different scopes, techniques and periodicity.