Answer to interview question - "What is the most important thing Object Oriented languages have contributed to software development?" ...sufficiently model the real world and problem at hand before jumping the gun. ...the success of graphical user interfaces like X and Windows. A GUI consists of several objects which have a behaviour on their own, something that OO is able to represent closely. ...polymorphism to match the logical way classification worked in real life -> code reuse ..limit project churn s tackexchange Tony Hoare introduced Null references in ALGOL W back in 1965 “simply because it was so easy to implement”, says Mr. Hoare. He talks about that decision considering it “my billion-dollar mistake”. http://www.infoq.com/presentations/Null-References-The-Billion-Dollar-Mistake-Tony-Hoare