This paper proposes a scalar analysis of polysemy of Russian verbal prefixes. The lexical entry remains constant throughout all uses of a given prefix: it relates the event, denoted by the prefixed verb, to a scale. The specific kind of transition denoted by the prefix is the source of the similarities in meaning. The structure, into which the prefix is inserted, varies and determines the scale along which the event is measured out, which may be a path (with verbs of motion), a scale of change, or the temporal trace of the event. It is demonstrated that the semantic differences go hand in hand with structural differences and that the meaning of a prefix is predictable based on the event structure of the verb it attaches to. If the verb lexicalizes a scale of change, the prefix must measure out the result, mapping the event onto a scale, which is the complement of the result. If the verb contains conflated material and is incompatible with a result, the only available position is above aspect, where the superlexical prefix measures out the time of the event. A direct object may serve either as the resultee undergoing a change of state or as the measuring scale (as in the case of spatial and consumption verbs). Many verbs are flexible, and then the prefix may take on different meanings and the structure depends on whether the event is interpreted as involving a change of state or an unbounded activity.