Sensory processing disorder occurs when one's body is unable to deal successfully with the information that it receives through its senses. The manifestation of this inability can appear in many ways, but, in order for the disorder label to apply, it must occur with frequency, intensity, and/or duration. Such dysfunction generally appears through a child's inability to either (1) read cues from his/her surrounding environment, which come as input through one or more senses; (2) consistently absorb and organize that sensory input; or (3) adjust his/her own behavior appropriately based on the effectively processed sensory input. Characteristics of SPD can appear as early as infancy, but atypical sensory processing does not automatically mean that a child will have difficulty learning or developing. Over time and with frequent reinforcement, a child can demonstrate growth in a range of areas and behaviors as a result of a successful course of therapy. This is the third in a series of articles examining the role of sensory integration in early childhood. This final article explores what is involved in assessing, diagnosing, and supporting a child who has a significant sensory issue. Sections include: (1) A Lens on SPD: When to Consider Assessment; (2) Occupational Therapists (OTs) and Assessment of Sensory Integration (SI); (3) Diagnosed: What Next?; (4) How OTs Believe SI-Related Techniques Can Help Teachers; and (5) Concluding Thoughts. [For the first article in this series, "Supporting Sensory-Sensitive Children in a Sensory-Intensive World," see EJ1303498. For the second article, "Classroom Solutions for Sensory-Sensitive Students," see EJ1303649.]