This dissertation explores case and verbal agreement in Icelandic. Case and agreement generally pattern together, but there are exceptional instances in which case and agreement come apart. In Icelandic, verbs agree with Nominative DPs. However, in some constructions, agreement with a Nominative is optional. In the standard account of case and agreement (Chomsky 2000), both types of features are determined simultaneously via the same syntactic operation. The standard theory, therefore, predicts that case and agreement should pattern the same way, and that neither should be optional. Moreover, based on fieldwork conducted at the University of Iceland, I present data that has not heretofore been reported. I argue that the likelihood of agreement depends on the type of construction. My research builds on other work which addresses optionality in Icelandic agreement (e.g. Sigurðsson and Holmberg 2008). This dissertation makes a substantial contribution to the literature on Icelandic agreement in that the rate of agreement across various types of constructions has not been examined. I illustrate that this type of optionality is not only robust, but also systematic. This dissertation contributes to the larger literature on case and agreement in several important ways. First, I argue for a departure from the standard proposal that case and agreement are established via the same syntactic operation. I propose that it is possible for the probe which assigns case to be in a relationship with a DP, even though the probe which establishes agreement is not in a relationship with that DP. Second, I provide empirical support for Multiple Agree. I argue that the survey findings reported in this dissertation provide evidence that a probe can enter into a relationship with more than one goal. Third, I provide empirical evidence for the optionality of Multiple Agree. I argue that agreement is optional only in constructions in which there is an item intervening between T and the Nominative, and Multiple Agree is, thereby, required in order for an agreement relationship to be established.