DRS (Decision Review System) Explained
A complete guide to the DRS in cricket: how it works, what technologies are used, when teams can use it, and the most famous DRS reversals in history.
The Decision Review System (DRS) is a technology-based system used in international cricket to allow teams to challenge on-field umpire decisions. Introduced in Test cricket in 2009 and progressively rolled out to ODIs and T20Is, DRS uses a combination of ball-tracking, edge detection, and video replay technology to adjudicate dismissal decisions more accurately than human eyes alone.
How DRS Reviews Work
In Test cricket, each team receives two unsuccessful DRS reviews per innings. When a player (batter or fielding team's captain) challenges an umpire decision, the third umpire reviews the decision using available technology. If the review overturns the on-field decision, the team retains their review. If the on-field decision is upheld, the team loses a review. With zero reviews remaining, no further challenges are possible for that innings.
Technologies Used in DRS
- Hawk-Eye (ball tracking): Reconstructs the trajectory of the ball using multiple cameras to predict whether it would have hit the stumps for LBW decisions.
- Ultra-Edge / Snickometer: A waveform showing sound spikes — any noise coinciding with the ball passing the edge of the bat indicates a possible edge for caught-behind reviews.
- Hot Spot: Infrared cameras that detect heat from friction — bat hitting ball leaves a white mark on the bat or ball in the infrared image.
- Real-Time Snicko: A faster version of the original Snicko that provides near-instant waveform data during reviews.
- Slow-motion video: Used for run outs (was the bat grounded before the bails were broken?), stumpings, and boundary decisions.
LBW Reviews Under DRS
LBW is the most common type of DRS review. Hawk-Eye checks three things: did the ball pitch in line? did the ball strike the pad in line? would the ball have hit the stumps? The '50-50 rule' applies: if ball-tracking shows the ball clipping the edge of the stumps, the original umpire's decision stands (benefit of the doubt to the umpire). Only if the ball would have clearly hit more than 50% of the stumps does DRS overturn an LBW decision.
Caught-Behind Reviews
Edge decisions (caught behind or caught at slip) are reviewed using Ultra-Edge and Hot Spot. Reviewers look for a sound spike on the waveform coinciding with the ball passing the edge, and/or a hot spot mark on the bat or ball. Neither technology is definitive alone — Ultra-Edge can pick up pad sounds; Hot Spot can miss faint nicks. The third umpire uses all available evidence to make a 'best available evidence' decision.
Frequently asked questions
Do all countries have DRS available?
No. Full DRS requires expensive technology (Hawk-Eye, Hot Spot, Ultra-Edge) and is not available in all cricket-playing nations. Some bilateral Test series between lower-ranked or emerging cricket nations may be played without the full DRS suite. In such cases, a simpler review system using only ball-tracking (and not Hot Spot or Ultra-Edge) may be deployed. ICC tournaments always use the full DRS suite.
What is the most famous DRS decision in history?
The most discussed DRS decision is arguably the 2019 Ashes Test at Headingley, where Steve Smith was given not out LBW after Australia's review showed the ball would have drifted outside leg stump — overturning what appeared a clear LBW on the replays. Smith subsequently scored a match-turning 77 not out, demonstrating how pivotal DRS decisions can be. Earlier, the controversy around the 'Umpire's Call' in the 2019 World Cup semi-final between India and New Zealand also generated widespread debate.