This chapter provides an in-depth investigation of both inter-speaker and intra-speaker variation in agreement with nominative objects and ECM nominative subjects. We both build on previous observations and report the findings of recent fieldwork. We show that in addition to a general dative intervention effect, clause boundaries, expletives, and singular datives are interveners for some speakers. We propose that Icelandic ECM clauses are either TP or vP and that the number feature on datives is visible for some speakers, contra previous research.