We provide the first treatment of typo-tolerant password authentication for arbitrary user-selected passwords. Such a system, rather than simply rejecting a login attempt with an incorrect password, tries to correct common typographical errors on behalf of the user. Limited forms of typo-tolerance have been used in some industry settings, but to date there has been no analysis of the utility and security of such schemes. We quantify the kinds and rates of typos made by users via studies conducted on Amazon Mechanical Turk and via instrumentation of the production login infrastructure at Dropbox. The instrumentation at Dropbox did not record user passwords or otherwise change authentication policy, but recorded only the frequency of observed typos. Our experiments reveal that almost 10% of login attempts fail due to a handful of simple, easily correctable typos, such as capitalization errors. We show that correcting just a few of these typos would reduce login delays for a significant fraction of users as well as enable an additional 3% of users to achieve successful login. We introduce a framework for reasoning about typo-tolerance, and investigate the seemingly inherent tension here between security and usability of passwords. We use our framework to show that there exist typo-tolerant authentication schemes that can get corrections for "free": we prove they are as secure as schemes that always reject mistyped passwords. Building off this theory, we detail a variety of practical strategies for securely implementing typo-tolerance.