This dissertation concerns the problem of free will. Particularly, it aims to shed light on the hierarchical theory of freedom, firstly presented by Harry Frankfurt in 1971. The preliminary hypothesis of this dissertation is that the problem of free will is appropriately understood in the terms suggested by the concept of grounding. I articulate the relevant claims about freedom in such terms, and I ask consequently what circumstance or condition can make the case that the agent is exercising freedom. Following this hypothesis, the goal of this work is to show that a revised hierarchical theory of free will constitutes a promising answer to the grounding question about freedom. I develop my arguments in support of this claim in two steps. First, I propose a critique of Frankfurt’s original theory of free will and its later developments. The objective of my analysis is to show that Frankfurt’s hierarchical theory of free will is ambiguous between different definitions of freedom and, hence, between different criteria for grounding freedom. At the same time, Frankfurt’s later proposals are not successful in the task of justifying free will. As a result, I argue that Frankfurt’s theory of freedom lacks a grounding element motivating the ability to exercise free will. Secondly, I develop an original hierarchical account of free will. Building upon my critique of Frankfurt, my proposal combines the main elements of Frankfurt’s original theory (hierarchical levels of desires, identification) with new elements from Frankfurt’s late reflection about the psychological structure of agents (caring, volitional essence). The main aim of my proposal is to equip a hierarchical theory of free will with a new grounding element, the volitional identity of the agents. In light of this, I conclude that free will is adequately defined as the agent’s wholehearted identification with psychic elements which belong to her volitional identity.