Continuous Descent Approach (CDA) or Optimized Profile Descent (OPD) is a method by which aircraft approach airports prior to landing. It is designed to reduce fuel consumption and noise compared with a conventional approach and involves maintaining a constant three degree descent angle during landing, until meeting the Instrument Landing System (ILS).
Continuous Descent Approach starts ideally from Top of Descent, i.e. at cruise altitude, and allows the aircraft flying its individual optimal vertical profile down to runway threshold. Some airports apply constraints to this individual optimal profile such as imposing a constant descent angle (e.g. 3 degree) or starting the CDA only after having left the holding pattern (e.g. London, at about 7000 feet) due to Air Traffic Management constraints.
CDA has historically had many critics who believed they were impractical at busy airports in crowded airspace. These criticisms were partially dispelled by the introduction of true CDA at several busy airports, including Los Angeles International Airport (LAX). The successful introduction of CDA in such environments was made possible by studying the descent characteristics of all common aircraft types and defining upstream limits that result in the heterogeneous arrival flows being spaced with safe separation and useful throughput. In addition, training given to air traffic controllers includes specific tactics for early identification and deconfliction of aircraft that would result in insufficient spacing on the arrival.
According to tests by Boeing and the FAA, CDA at a single airport can save millions of pounds of fuel, and reduce atmospheric carbon dioxide emissions by millions of pounds. 
Air traffic control in the United Kingdom will give a distance to run to pilots at major UK airports. With this figure and the aircraft's altitude, pilots can calculate their rate of descent and perform a Continuous Descent Approach.
The UK, and London Heathrow Airport in particular, claim to be the world's leaders in Continuous Descent Approach; however, their definition of CDA only indicates a continuous descent from 6000' altitude, not from Top of Descent, and in fact the published arrivals at major UK airports contain numerous "descent planning" sections indicating level segments at lower altitudes, increasing fuel consumption and noise.
Due to the negative association with the term CDA, new procedures introduced at LAX and other US airport are instead called Optimized Profile Descent (OPD).
The spanish Developement Ministry has studied to force airlines to land using CDA method in all major airports. The Defence Ministry has also opened the militar airways to the commercial planes.