Respiratory muscle ultrasound is used to evaluate the anatomy and function of the respiratory muscle pump. It is a safe, repeatable, accurate, and non-invasive bedside technique that can be successfully applied in different settings, Mastery of this technique allows the intensivist to rapidly diagnose and assess respiratory muscle dysfunction in critically ill patients either mechanically ventilated or non mechanically ventilated.This paper provides an overview of the basic and advanced principles underlying ultrasonography of  the diaphragm. We review different ultrasound techniques useful for monitoring of the respiratory muscle pump and possible therapeutic consequences. Ideally, respiratory muscle ultrasound is used in conjunction with other  clinicolaboratory components of critical care to obtain a comprehensive evaluation of the critically ill patient.  Introduction:Over the last 25 years, numerous studies have supported the advantage of ultrasonography (US) in the assessment of diaphragmatic function. Various ultrasonographic methods, such as measurement of diaphragmatic excursions by two dimensional (BD)[1,2] or M-mode[3,4] and changes in diaphragm thickness during inspiration[5], have been proposed. In this review, we report the role of diaphragmatic ultrasound in mechanically ventilated patients versus non mechanically ventilated patients with diaphragmatic dysfunction in Intensive Care Unit Aim of the Work  This work aimed to illuminate the role of diaphragmatic ultrasound in mechanically ventilated patients versus non mechanically ventilated patients with diaphragmatic dysfunction in Intensive Care Unit  Methods:  100 Patients were allocated into two main groups: § Group I : Non mechanically ventilated patients § Group II : Mechanically ventilated patients § Each group was divided into three subgroups § Subgroup I : Respiratory failure patients § Subgroup II : Stroke patients § Subgroup III : Sepsis patients Diaphragmatic ultrasound was done for all allocated patients and different parameters  as diaphragmatic thickness (DT), diaphragmatic thickness fraction (DTF) and diaphragmatic excursion (DE) were measured on the first day of admission and on the seventh day of admission. The results  Evaluation of diaphragmatic thickness (DT)and diaphragmatic thickness fraction (DTF) : as percentage from the formula: (thickness at end inspiration– Thickness at end-expiration)/Thickness at end Expiration * 100.  and diaphragmatic excursion (DE) are easily obtained and comparable parameters with clinical and laboratory parameters  to evaluate either mechanically ventilated or non mechanically ventilated critically ill patients Conclusion: Diaphragmatic ultrasound parameters are useful in conjunction with other  clinicolaboratory components of critical care to obtain a comprehensive evaluation of the critically ill patient.