While effective, the impacts of GLP-1RAs on glycaemic control and body weight are variable, usually for uncertain reasons. There is a lack of reliable information relating to the effects of GLP-1RAs to slow gastric emptying (GE) and induce gastrointestinal symptoms, in part because these have often not been evaluated using optimal methodology. ‘Short-acting’ GLP-1RAs (i.e. exenatide BID and lixisenatide) slow GE markedly and, contrary to previous thought, ‘longer-acting’ GLP-1RAs (now shown for liraglutide, exenatide QW and subcutaneous semaglutide) also have a durable and significant effect. GE is delayed (i.e. gastroparesis) in 30-50% of individuals with longstanding, poorly controlled type 1 or type 2 diabetes, but in well controlled type 2 diabetes, GE is often abnormally rapid. GE is a major determinant of the glycaemic response to carbohydrate-containing meals; in insulin treated diabetes gastroparesis may predispose to hypoglycaemia. Gastrointestinal symptoms occur frequently in diabetes and/or obesity. When evaluating GLP-1RAs GE and gastrointestinal symptoms should be assessed optimally. Scintigraphy remains the ‘gold standard’ technique to measure GE, but stable isotope breath tests, which are simple and not associated with a radiation burden, are a validated alternative. The paracetamol absorption test has substantial limitations and its use in isolation should be avoided. In clinical trials of GLP-1RAs gastrointestinal symptoms have been, and are usually evaluated solely using participant ‘self-report’ which is unreliable, despite the availability of simple, validated measures. It has recently been appreciated, albeit based primarily on anecdotal reports, that ‘longer-acting’ GLP-1RAs may lead to prolonged retention of gastric contents, evident at endoscopy/surgery. Slowing of GE and the induction of gastrointestinal symptoms are likely to be relevant to weight loss induced by GLP-1RAs, but their importance is uncertain. Measurements of gastric emptying and gastrointestinal symptoms, using precise techniques, should be incorporated widely in future clinical trials of GLP-1RAs.