This chapter provides an overview of adverse effects of immunization and missed opportunity immunization. An adverse event following immunization (AEFI) is any untoward medical occurrence which follows immunization and which does not necessarily have a causal relationship with the usage of the vaccine. The adverse event may be any unfavorable or unintended sign, abnormal laboratory finding, symptom or disease. All serious or severe AEFI is to be immediately notified by the first person who identifies the event. This person should notify the case to the nearest government primary health center (PHC) or community health centers (CHC) and/or the district immunization officer (DIO) by the quickest means of communication. All notified AEFI should be documented on a case reporting form (CRF) and submitted at next level as soon as possible. Causality assessment is the systematic evaluation of the information obtained about an adverse event following an immunization to determine the likelihood that the event might have been caused by the vaccine(s) received. Indian Academy of Pediatrics in collaboration with its Kutch branch has started infectious disease surveillance and AEFI reporting system for reporting serious AEFI. The objectives of the project are to develop an early warning system for pediatric infectious diseases in India, to generate data on burden of infectious diseases in India and to generate data on serious AEFI. Missed opportunity immunization is defined as any contact with a health service that did not result in an eligible child or woman receiving the needed vaccines.