Oscilloscopes, previously also known as oscillographs, and scope or o-scope, CRO, or DSO, are a type of electronic testing instruments that can graphically display varying signal voltages, usually as a calibrated two-dimensional plot.

O-scopes are an important tool in any electrical engineer's lab. They allow you to see electric signals as they vary over time, which can be critical in diagnosing why your 555 timer circuit isn't blinking correctly, or why your noise maker isn't reaching maximum annoyance levels. The main purpose of an oscilloscope is to graph an electrical signal as it varies over time. Most scopes produce a two-dimensional graph with time on the x-axis and voltage on the y-axis.

In addition to those fundamental features, many scopes have measurement tools, which help to quickly quantify frequency, amplitude, and other waveform characteristics. In general a scope can measure both time-based and voltage-based characteristics.