Heisenberg's Uncertainty Principle

Werner Karl Heisenberg (1901-1976) stated in 1927 that certain specific pairs of variables cannot be measured simultaneously with high accuracy. Most importantly, he pointed out that within an atom, it is possible to measure the position, or the momentum, of a subatomic particle such as an electron. However, it is not possible to measure both of them at the same time, because the measuring process interferes to a substantial degree with what is being measured.

Further info :

University of Winnipeg, CA - The Uncertainty Principle
http://theory.uwinnipeg.ca/physics/quant/node7.html#SECTION002760000000000000000

Stanford Linear Accelerator Centre - Quantum Mechanics Theory
http://www2.slac.stanford.edu/vvc/theory/quantum.html

St.Andrews University, Scotland - Quantum Mechanics History
http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/The_Quantum_age_begins.html