Understanding MCS is an accessible introduction to Multiple Chemical Sensitivity, Environmental Illness, and mast cell related disorders that is aligned with the DSM-5. Written primarily for therapists--social workers, mental health counselors, expressive therapists, psychologists--it is intended to inform all clinicians and healthcare professionals. Understanding MCS offers DSM-5 assessments and coding for these hard to recognize and often difficult to manage chronic diseases as well as providing client-focused supports, guidelines for reasonable accommodations, insights regarding treatment planning and interventions, and suggestions for alternatives in counseling practice in order to best serve individuals living with these chronic illnesses.