As of July 2013, estimates of more than 170 million debris smaller than 1 cm (0.4 in), about 670,000 debris 1–10 cm, and approximately 29,000 larger pieces of debris are in orbit.
Space debris poses a serious threat to operational satellites in space and collisions are a real possibility and can prove fatal to the mission, and possibly human lives.
Feasible technologies to deal with the space debris problem do exist.
A December 2009 conference sponsored by NASA and DARPA (the Defense Advanced Research Projects Agency), featured many proposed solutions, including large orbiting shields to catch small debris, ground-based lasers to ablate the front side of debris to deboost it, and active spacecraft to capture large debris items and drag them down to atmospheric entry.
Source: Klinkrad, Heiner, and Johnson, Nicholas, “Space Debris Environment Remediation Concepts,” NASA-DARPA International Conference on Orbital Debris Removal, Chantilly, VA, 8-10 December 2009.
There is also the electroDynamic debris eliminator, which is a low-cost solution, efficient and weighs less.
Considering how serious a problem space debris (especially those in LEO) poses to operational satellites, and given that feasible solutions do exist, why hasn't any active space debris removal system been implemented so far (to my knowledge)?