Acute flaccid paralysis is a diagnostic challenge in the emergency department. Periodic paralysis syndromes are characterized by recurrent episodes of flaccid hyporeflexic paralysis in association with potassium abnormalities. Periodic paralysis with hypokalaemia may be genetic, secondary to systemic hypokalaemia or associated with thyrotoxicosis. Genetic syndrome result from mutations in sodium (SCN4A) or calcium (CACNA1S) channels, inherited in autosomal dominant pattern. Diagnosis is established by demonstrating recurrent nature, family history and abnormal serum potassium during an episode. Thyrotoxic periodic paralysis is often sporadic but possibly has a genetic predisposition. Presence of thyrotoxicosis and hypokalaemia during an episode confirms the diagnosis. Management of acute episode is by cautiously correction of potassium abnormality. Long term therapy depends on the cause. Pathogenic mechanisms, differential diagnosis and treatment principles are discussed.