This research reviews the existing Tax Planning (TP) measures and explores the consistency of UK firms’ engagement in TP; evaluates incentives for TP and its value relevance in a signalling theory framework; and studies corporate governance effects on TP for the firms in an institutional theory framework. It analyses a unique set of 1,482 hand-collected firm-year observations and proposes ‘undisclosed TP’ as a new TP measure. It finds that firms consistently engage in TP and their TP disclosures have improved; internationally oriented firms do not engage in TP to save taxes; risky firms, firms with low operating cashflows and growing firms, however, do not engage in TP to arrange funds internally – so they signal their non-engagement in TP to the market. Further findings confirm public awareness and market valuation of firms’ TP engagements. Boards’ tax affiliations result in reductions in tax payments (expenses) for strongly (weakly) governed firms. Professional accountancy qualifications on the board result in significantly higher tax payments for weakly governed firms. The auditors’ provided tax services (institutional ownership) result in higher tax payments for weakly (strongly) governed firms suggesting supplementary (complementary) role of auditors (institutional ownership) for the internal governance on TP. This research concludes that there is a need for further TP disclosures to reduce the information asymmetry associated with negatively valued TP activities; recommends auditors’ involvement in TP services; and recommends tax affiliates on the board to bring tax savings in a strongly governed environment. The current study’s findings have important theoretical and practical implications. / University of Bradford