To prevent unauthorized access to their smartphones, users can enable a "lock screen," which may require entering a PIN or password, drawing a pattern, or providing a biometric. We present the results of two studies that together offer a detailed analysis of the smartphone locking mechanisms currently available to billions of smartphone users worldwide. An online survey (N=8,286), conducted in eight different countries, sheds light on people's reasons for choosing their screen lock method and demonstrates significant crosscultural differences in attitudes towards this subject. In a separate monthlong field study (N=134), we studied how existing lock screen mechanisms provide users with distinct tradeoffs between usability and security, identifying areas where both could be improved.