This thesis provides a broad study of the tenurial, administrative and political history of Yorkshire in the first century of English feudalism. It begins by providing a new and more precise chronology for the Norman conquest of Yorkshire and illustrates the importance of castleries and hundreds in the process of take-over. In Chapter 2 the thesis reveals that in the fifty years following the Domesday survey the Normans extended the system of compact lordships based upon castleries, hundreds and hundredal castleries in order to bring the more remote parts of the county under control and to provide protection for its borders. The new men placed in control of these lordships played a vital role in the integration of Yorkshire within the royal system ofjustice and administration. Attention is then paid in Chapter 3 to the scale and pattern of Norman sub-enfeoffment in the period 1086 x 1135. The study throws new light on both the purpose of the system of military service introduced by the Normans and the reasons for the rapid expansion of monasticism in Yorkshire after 1100. Chapter 4 illustrates how after 1135 royal control over the local administration of Yorkshire disintegrated in the face of the political difficulties of King Stephen and the growing power of William earl of York, and Chapter 5 examines how King David of Scotland exploited Stephen's weakness in the northern England to extend his influence within the area. Chapter 6 considers the nature of some of the new enfeoffment tenancies recorded in the 1166 inquest and elucidates the reasons behind the reluctance of magnates to acknowledge their existence and pay scutage upon them. And finally, the thesis concludes in Chapter 7 with a major re-assessment of the nature and strength of lordship and the emergence of property right in the first century of English feudalism.