The diagnosis of growth hormone deficiency (GHD) was essentially a clinical one prior to the advent of radioimmunoassay in the mid-1960s. From this point on both clinical and biochemical serum GH responses to a variety of provocation tests were used to define the condition. The definition of an adequate GH response to stimulation has changed over time, initially being <3 microg/l and gradually increasing to 10 microg/l. Over this period, GH assays became more sensitive and specific, and assays for IGF-1 and IGFBP-3 were developed. Detailed neuroimaging also became widely available and genetic aetiologies for GHD identified. Apart from a number of clear genetic causes for GHD, the diagnostic gold standard remains elusive. However, making the correct diagnosis has significant benefits to the child, guiding both future investigations and management. In this chapter we discuss the importance of taking into account all available evidence when making the diagnosis of GHD including clinical examination, detailed auxological measurements, bone age, molecular analysis, biochemical measures and neuroradiological assessment.