Flying

Flying is a evasion ability that makes creatures without flying unable to block creatures with flying. It has been in Magic since the original Alpha set. Flying appears on over 900 different cards and is scattered wildly on the color wheel, mostly being found on blue and white cards, but also appearing in Black and to a lesser extent Red. Green rarely has flying creatures, but is the best color at destroying them (i.e. Wing Snare).

From the Comprehensive Rules

• 502.4. Flying
  • 502.4a Flying is an evasion ability.
  • 502.4b A creature with flying can't be blocked except by creatures with flying and/or reach. A creature with flying can block a creature with or without flying. (See rule 309, "Declare Blockers Step" and rule 502.70, "Reach.")
  • 502.4c Multiple instances of flying on the same creature are redundant.