Comorbidity is an area of great importance in autism research. One of the most common co-occurring problems with autism spectrum disorder (ASD) is challenging behaviors. The aim of this chapter is to discuss challenging behaviors in the context of ASD. We need to define challenging behaviors to better understand what constitutes a challenging behavior. One of the difficulties in assessing challenging behaviors is in deciding what what is a challenging behavior and what is not a challenging behavior. Often challenging behaviors are subjective. They cause problems to an individual themselves or others around them, and then become challenging. One behavior may constitute a challenging behavior for one individual with ASD or their caregivers and may not be a behavior of concern for another individual with ASD and their caregivers. In order to provide the best treatment for individuals with ASD, it is imperative that a thorough assessment is conducted. Many methods have been developed in order to assess challenging behaviors. This chapter will review the measures that have been used to assess the maintaining variables of challenging behaviors. These are the measures used for the functional assessment of challenging behaviors. An additional aim of this chapter is to provide a review of the types of scales that are available to identify whether challenging behaviors are present.