Developmental delay (DD) is a commonly encountered clinical problem. Nearly most children with neurodevelopmental disorders present with developmental delay. The ultimate diagnosis may take months or even years to be confirmed. The final verdict may be purely developmental with a benign outcome, or there might be slow deterioration ending up in a more sinister cause like neurodegenerative diseases. At times, the developmental course might plateau off with the final developmental outcome underachieved. With standardized assessment at a later age, these children could then be labeled as suffering from "mental retardation (MR) or "intellectual disability" (ID). Thus, the crucial issue during the process of making the final diagnosis for any child presenting with developmental delay is when can one decide that an etiological diagnosis can confidently be made during the phase of development. When can one be really sure that there is no underlying organic cause to account for the developmental delay. Often than not, the clinician is faced with the dilemma of how exhaustive one should persue in investigating a case of DD, and when to stop investigating further. It is very important for us to develop a logical and practical approach in managing these DD children at the "first" presentation and to establish ones clinical judgment to find the "Best timing" for referral or diagnostic workup. The first step to the approach is the awareness and the differentiation of Partial Developmental Delay (ParDD) from Global Developmental Delay (GDD). We aim to provide the readers an understanding of a practical and evidence based approach for the diagnostic evaluation of a child with DD/MR. We hope this approach can help to improve the diagnostic yield of DD/MR.