There has been considerable debate about the merits of the different legal instruments used to achieve public policy goals. This thesis investigates the effectiveness of different instruments that a government can use to secure habitat protection goals. Comparison is made between two different countries: the United Kingdom, with specific reference to Scotland, and Spain, with specific reference to the Autonomous Community of Madrid. The first phase of the research was the development of a theoretical framework, based on a review of the literature, to identify a suitable system of classification for the instruments. This facilitated the identification of their strengths and weaknesses and the derivation of a set of criteria which could then be used to test the instruments in Scotland and Madrid. The research was principally based on a desk-study of the literature, legislation and case-law. Information about the operation of the legislation in each of the countries under study was completed with qualitative data from unstructured interviews. The study focuses on those instruments which play a major role in habitat protection in each of the two countries studied. This allowed a comparative evaluation to be made of the main instruments, enabling a comprehensive understanding of the strengths and weaknesses of the instruments. It was found that no single mechanism can meet the range of criteria identified and that a combination of instruments can respond more appropriately to the protection of habitats. However, any policy utilised requires reliance on some form of regulation to be effective.