This thesis examines the process of discernment as described by three Spanish Catholic mystics of the sixteenth century: Ignatius Loyola (1492-1556), Teresa of Avila (1515-1582), and John of the Cross (1542-1591). Discernment refers to the process of distinguishing those things which are of God from those which are not. This thesis demonstrates that all three of these mystics had specific criteria by which they evaluated their mystical experiences to determine whether or not they were from God, shows that there are important commonalities between their discernment criteria, and argues that these criteria in large part derive from their social and historical setting.