Exponential growth bias is the pervasive tendency to linearize exponential functions when assessing them intuitively. We show that exponential growth bias can explain two stylized facts in household finance: the tendency to underestimate an interest rate given other loan terms, and the tendency to underestimate a future value given other investment terms. Bias matters empirically: More-biased households borrow more, save less, favor shorter maturities, and use and benefit more from financial advice, conditional on a rich set of household characteristics. There is little evidence that our measure of exponential growth bias merely proxies for broader financial sophistication. WHAT DRIVES HOUSEHOLD financial decisions? The canonical economic model assumes that consumers choose to consume, borrow, or save based on their preferences , their expectations, and the costs and benefits of borrowing and saving. A growing body of work applies insights from psychology to enrich specifications of three of the model's key pieces: preferences, expectations, and problem-solving conditional on parameter values. 1 In this paper, we bring psychological evidence to bear on a fourth specification issue: How consumers perceive the costs and benefits of borrowing and saving. We begin by tying together existing and new evidence on these cost perceptions to show that most consumers err systematically when given information commonly available in the market. On the saving side, consumers display future value bias: a systematic tendency to underestimate a future value given