A collocation can be defined as a sequence of words which co-occur more than would be expected by chance. Collocations impose constraints on how words can be used together. An idiom is an expression whose meaning cannot be deduced from the literal definitions and the arrangement of its parts, but refers instead to a figurative meaning that is known only through common use. In this paper I will attempt to expand on these notions. For further clarification, I will compare and contrast some examples from English and Persian.