Neuromuscular weakness occurs very commonly in critically ill patients and is a common cause of failure to wean from the ventilator and decreased limb movements. A rational approach to weakness will help to identify the causes of neuromuscular weakness in the intensive care unit (ICU). The most common acquired causes of weakness in the critically ill patient in the ICU are critical illness neuromyopathy (CINM) including critical illness polyneuropathy and critical illness myopathy. The majority of survivors with CINM have persistent functional disabilities and a reduced quality of life. There are needs for new therapeutic strategies to prevent or minimize CINM in critically ill patients. This article will focus on the diagnostic strategy of CINM, identified risk factors, several proposed pathomechanisms and their clinical implications.