Today's time and frequency standards range from the most sophisticated reference standards to the smallest oscillator for handheld radios. The technical requirements and technologies needed are different for the various applications but they derive from similar physical concepts. These different technologies can be categorized into four major areas, reference standards, mobile systems, man-portable (handheld) and space systems. These areas are the core areas of time and frequency standard applications and different areas of technology are needed to address them. This presentation discusses the time and frequency standards used or available for these areas.