The past decade has witnessed a significant increase in cross-border capital flows and a pronounced shift in their composition towards portfolio investment, with much of the capital under management by mutual funds or "collective investment institutions" resident in OECD countries, as well as in offshore tax-free environments. These developments motivate countries to review and monitor the operation of their tax rules in this area, notably the interaction of domestic tax rules with tax systems and practices in other countries, to ensure that policy goals are being addressed. This book analyses the taxation of cross-border portfolio investments by means of collective investment institutions. Possible tax distortions specific to the area of collective investment institutions are identified for a representative group of OECD countries. The analysis and calculations contained in the book, while illustrative and not capturing the full richness of the possible permutations under tax regimes, draw out key tax distortions, neutrality conditions and policy options.