Even though word-formation skills may greatly enhance vocabulary acquisition, they are not often emphasized in language teaching or research. Since there are no validated methods to assess learners’ derivational skills, three different test types were developed and their applicability is discussed. This study also investigates the relationship between the written performances of Finnish learners of English and their derivative skills. The results showed quite expectedly that there is a fairly strong relationship between written skills and word formation knowledge. However, the fi ndings suggest that learners need to have reached level A2 in English before they can start to make use of word formation as a linguistic resource. The comparison between different test types gave promising results as to how word formation could be assessed.