This chapter defines deafblindness and looks at different terms used in connection with the condition. I consider the prevalence of deafblindness. The chapter outlines some of the causes of deafblindness, distinguishing between whether on the one hand it is congenital or has an early onset, or on the other hand whether it is acquired. I examine implications for a child who is deafblind of whether hearing impairment and/or visual impairment are congenital (or appear early in development) or are acquired. The chapter then considers the identification and assessment of deafblindness, looking at the specialists who assess the degree of deafblindness and at the assessment of communication skills and physical and social skills.