In this dissertation I examine Vergil's use of laetus (and laetitia, laetor, and inlaetabilis) in the Aeneid; for context, I also present the use of the laet- stem in earlier poets and in the Georgics and Eclogues. The two basic uses of laetus in Latin literature indicate human emotion (joyful) and agricultural lushness (fertile). I argue that the laetus-emotion in the Aeneid is one of hopeful joy: joy in that the emotion is vivid, positive, and thrilling, and hopeful in that the source of the joy is hope for the future. This hope is usually created by a reversal (or perceived reversal) of fortune; an event (like an omen, victory in battle, etc.) causes an individual with low expectations of success to have high expectations of success. Nevertheless, this hope may be disaster-prone, as Lyne argues: this hopeful joy often results in disaster. The laetus-emotion is a thrill, a shock of sensation, and it does not refer to long-term contentment. In addition, this thrill of sensation is often accompanied by physical excitement: a laetus individual is often loud and jubilant in his physical exultation. In my introduction, I offer background information on other scholars' studies of laetus and on the use of the word in other Latin authors. In Chapter Two, I treat the instances of laetus in the Aeneid outside Book 5 that express human emotion, in the context of fields that I identify (setting out, arrival, battle, prayer, founding). In Chapter Three I discuss the uses of laetus in Book 5 of the Aeneid; I separate them in this way because the uses in Book 5 neatly demonstrate my arguments regarding laetus in the eleven other books. In Chapter Four I discuss all the instances of laetitia, laetari, and inlaetabilis in the Aeneid. In Chapter Five I treat the laet- stem in the Georgics and Eclogues, and in Chapter Six, the analogous agricultural use in the Aeneid. In Chapter Seven I discuss the uses of laetus in the Aeneid that do not neatly conform to any of my categories, and I offer suggestions as to their interpretation.