The goal of this clinical trial is to develop and test a brief online intervention to reduce alcohol and cannabis misuse and improve healthy relationship skills among young adult couples. The main questions it aims to answer are:

• Will the intervention be feasible and acceptable to young adult couples?
• Will the intervention demonstrate initial efficacy in reducing risky substance use and increasing relationship functioning?

Eligible couples will complete a virtual baseline session and be randomized to intervention condition (online intervention with 3-5 weeks of self-paced modules) or control condition (no intervention). Couples will complete two follow-up surveys (post-assessment - approximately 5 weeks after baseline, 3-month). Couples in the control condition will be offered the intervention after 3-month follow-up.

Researchers will compare intervention and control groups to see if there there is a difference between the groups on substance misuse and relationship functioning at post-assessment and 3-month follow-up.

Inclusion Criteria:

• Both partners in the couple are 18-29 years old
• In a committed romantic relationship (e.g., dating seriously, cohabiting, married) with each other for at least 3 months
• Face-to-face contact their partner at least 5 days per week
• Live in Washington State
• Have a valid email address and access to a cell phone
• Report alcohol and cannabis co-use at least three times in the past month
• Willingness to: complete online surveys during the allotted time frames, receive text messages and emails from the project, complete a baseline session, and participate during the same time period as their partner

Exclusion Criteria:

• Couples who endorse any perpetration or receipt of severe interpersonal aggression during screening will be excluded and provided with resources