Autism Spectrum Disorders (ASD) is a brain disorder that affects an individual's social skills and behaviours. While information surrounding the diagnosis of children with ASD has increased in recent years, there is a lack of information about the diagnostic process for adults with ASD. Although early diagnosis is the goal, clinicians often encounter adults with undiagnosed ASD. Further, the diagnosis of adults suspected of having ASD is complex. For instance, many of the available diagnostic tools are intended for children, or are limited for use up to 22 years of age. In addition, many tools involve recalling events from the individuals past, which may be difficult for the individual, or their parent, to remember correctly. I aim to determine the most accurate diagnostic tools for identifying adults with ASD. I will administer various diagnostic tests to adults with and without ASD. I will compare the tests on their ability to accurately identify individuals with ASD, as well as their ability to rule out individuals that do not show symptoms of ASD. As a result, I will be able to pinpoint which tools are suitable for diagnosing adults with ASD. Another goal of this research is to determine the best combination of diagnostic tools that lead the most accurate diagnosis. For instance, I will discover whether certain tools should be used alone versus in combination with others. The results of this study will help inform practitioners, clinicians, and educators on the most effective process for diagnosing adults with ASD. More broadly, accurate and efficient diagnosis will allow individuals with ASD access to the necessary support to improve their work and personal relationships, and overall quality of life.