Gambling disorder is today a common disorder causing lives to shatter. As this disorder activates the reward system and increases the need of dopamine, recovering from gamling disorder can be hard. Research has shown that gambling addiction appears to have similarities to substance abuse, but no substance has been named. Therefore, this systematic review discusses similarities and differences between gambling and opioid addiction. The aim is to compare behavior and neurological correlates of gambling and opioid addiction tocreate a better understanding of the relationship between these addictions and hopefully help improve treatments. My expectation was that I would find behavioral and neurological similarities between opioid and gambling addicts. To investigate this, I conducted a systematic search across databases. I compared six studies which all used a task to investigate decision-making ability and brain-scanning to investigate neural correlates. Both gambling and opioid addicts show decreased activation in the orbitofrontal cortex, which plays a central role in decision-making. Regarding task performance, some studies showed addicts to perform worse than controls, suggesting that addicts show an impaired decision-making ability compared to controls. This systematic review contributes to the literature within addiction, but to draw a conclusion that gambling and opioid addicts suffer from similar decision-making impairment further research is needed.