Trust is critical to the economic, political, and social coordination and cooperation underpinning society, yet we find ourselves in a perceived “crisis of trust,” particularly in institutions such as government, business, the media, and NGOs. But what is trust, and why is it thought to have declined in recent years? This chapter considers the problem and the nature of trust in both an interpersonal and an institutional (collective) form, asserting that trust involves a three-part relationship between a trustor, a trustee, and some domain of behavior wherein the trustee’s behavior is perceived to encapsulate the interests of the trusting party. Choosing to trust is not a risk-free endeavor for the trusting party, as it involves the acquisition of (usually imperfect) information about the trustworthiness of a trustee that is then used to form a justified true belief about the trustee’s trustworthiness as a basis of a trustor’s decision about whether to act in a given situation. By clarifying the notion of trust in its interpersonal and institutional forms, this chapter lays the foundation for considering the relationship between trust and distributed ledgers, including blockchains, in the following chapter.