One difference is that they are used for all control structures: a = b ifTrue:[ ... ] a = b ifFalse:[ ... ] a = b ifTrue:[ ...] ifFalse:[ ... ]

One difference is that they are used for all control structures: a = b ifTrue:[ ... ] a = b ifFalse:[ ... ] a = b ifTrue:[ ...] ifFalse:[ ... ] [ a = b ] whileTrue: [ ... ] And "control structures" aren't actually control structures that are built into the language. They are just messages sent to blocks (closures). That includes iteration, so a for "loop" is: 1 to:10 do: [ :i | ... ] Which is just a shorthand for (1 to:10) do:[ :i | ... ] With (1 to:10) creating an Interval object, a virtual collection that understands the normal collection iteration protocol (do:, collect:, select:, reject:, inject: ). OO + Closures + keyword syntax. An elegant and powerful combination.