This thesis investigates the relationships between ownership structure and agency costs, executive compensation and earnings management in large private firms in the UK. The study employs a sample of 1223 firms and data is collected from the FAME database and Annual Returns submitted to Companies House. The study uses a cross-sectional data set, with data from sample firms 2013 financial reports, and employs univariate tests and OLS and 2SLS regressions to test the proposed hypotheses. The findings of the agency cost study are that high and low agency cost firms differ significantly in their managerial ownership and, to a lessor extent, in their ownership concentration. Agency costs, measured by the expense ratio, are negatively related to managerial ownership. Measured by asset utilisation, agency costs are non-linearly related to managerial ownership, declining and then rising with increasing managerial ownership. There is limited evidence of a monitoring effect associated with concentrated ownership. The findings of the executive compensation study are that compensation is cubicly related to managerial ownership and is lower where ownership is highly concentrated. Compensation is sensitive to accounting performance. Pay-performance sensitivity is lower in firms in which the Managing Director is a majority shareholder and in firms in which ownership is highly concentrated. The findings of the earnings management study are that income increasing discretionary accruals, considered in isolation, are non-linearly (U-shaped) related to managerial ownership. Firms in which managerial ownership is low exhibit significantly higher income increasing discretionary accruals when firms experience a cash-flow loss or decline. A similar result is obtained using both measures of poor cash-flow performance, however the magnitude of the coefficients indicates that the effect is stronger when a cash-flow loss is experienced. In general the results show that both low and high levels of managerial ownership are associated with significant agency costs in large private firms and that concentrated ownership performs a limited monitoring role in these firms.