The aim of this review is to present the current options for cardiac output (CO) monitoring in children undergoing cardiac surgery. Current technologies for monitoring identified were a range of invasive, minimally invasive, and non-invasive technologies. These include pulmonary artery catheter, transoesophageal echocardiography, pulse contour analysis, electrical cardiography, and thoracic bioreactance. A literature search was conducted using evidence databases which identified two current guidelines; the NHS Greater Glasgow and Clyde guideline and Royal College of Anaesthetics Guideline. These were appraised using the AGREE II tool and the evidence identified was used to create an overview summary of each technological option for CO monitoring. There is limited evidence regarding the accuracy of modalities available for CO monitoring in paediatric patients during cardiac surgery. Each technology has advantages and disadvantages; however, none could be championed as the most beneficial. Furthermore, a gold standard for CO monitoring has not yet been identified for paediatric populations, nor is it apparent whether one modality is preferable based on the available evidence. Additional evidence using a standardised method for comparing CO measurements should be conducted in order to determine the best option for CO monitoring in paediatrics. Furthermore, cost-effectiveness assessment of each modality should be conducted. Only then will it be possible for clear, evidence-based guidance to be written.