Problematic alcohol use (PAU) is a common, yet serious, health issue among college students that can lead to negative outcomes such as assault, alcohol use disorders and even death. Although PAU often occurs in group settings, bystanders frequently choose to do nothing when witnessing their peers engaging in PAU. Little is known about why, when, and where this lack of response occurs because there are no measures for assessing and understanding bystander behavior in the context of PAU. This information is critically needed before effective interventions for bystanders can be developed and evaluated. As part of our existing NIAAA funded R15 (1R15AA028910-01), our team has developed the Bystander Intervention for Problematic Alcohol Use Model (BIPAUM). We have also developed and tested a battery of scales to assess some constructs across this model. However, additional constructs were identified in our qualitative work and corresponding scales need to be developed and tested. We propose the following specific aims: 1) Develop measures of Notice and Interpret, Identify Intervention Strategy, Assess Support, and Assess Outcomes, 2) establish the psychometric properties and validate the remaining scales, and 3) establish measurement invariance of the scales. To achieve Aim 1, the team will utilize existing data from our n=20 focus groups and n=9 interviews (n=79 diverse college students) to create items to include in the additional scales. Scales will then be vetted by obtaining feedback from students (n=80) and content experts (n=10). To achieve Aim 2, a sample of n=800 students will be recruited via Prime Panels to complete a 20-minute survey containing these scales. These data will be used to conduct item analysis, exploratory factor analysis, and reliability analysis. Another sample of n=800 students will be utilized to conduct confirmatory factor analyses, convergent, discriminant, and criterion validity checks. Finally, to achieve Aim 3, we will use a multiple-group CFA framework to test validity and measurement invariance of the BIPAUM. The same sample as Aim 2 will be used to accomplish Aim 3. Our research team has expertise in measurement development, bystander behaviors, and PAU and is well suited to carry out this work. Once all measures that correspond with the BIPAUM have been developed and validated and the model can be tested, MPIs will seek R01 funding to conduct a longitudinal analysis of the BIPAUM, in addition to designing and evaluating a novel bystander intervention program for PAU. An intervention that taps into the social nature of bystander intervention and drinking and is theoretically informed has the potential to reduce PAU and its detrimental consequences among college students.