The diaries of Pedro de Rivera and the Marques de Rubí, written in the eighteenth century during inspections of the far northern frontier of New Spain, are crucial documents for studying and understanding the Spanish presence on the frontier of what would one day be Texas. Rivera’s diary, previously unavailable in English translation, and the heretofore unknown Rubí diary are both presented here, carefully placed in historical context by Jackson and Foster. Because of Spain’s tenuous hold on the distant frontier, Rubí and Rivera saw it as an imaginary possession—the king’s domain in name only. No other military visits to the frontier in this era rivaled those of Rivera and Rubí in scope, organization, or execution. They were significant fact-finding commissions, authorized by the Spanish king, that resulted in extensive reports, broad recommendations, and tangible changes in military regulations for the presidios on the far northern frontier. To understand Texas and its adjacent provinces at this formative time, students and scholars of the Borderlands must examine the records left by these two military expeditions. These remarkable documents contain fascinating insights into the early Spanish road systems, the early towns and missions, the Indians, and the flora and fauna. Each diary has an introduction, and detailed route maps and annotations are provided. Following the diaries and related documents, each inspection is assessed in depth. These two diaries, and the editors’ careful annotation and analysis, provide significant new material which will help further understanding of Spanish Texas, the people who inhabited it, and its influence on the Texas of today.