This study focuses on the impact of different variables on the nature of language performance in the context of task-based instruction. Characteristics of tasks are discussed, and then a framework is offered that can organize the nature of task-based instruction and relevant research. The framework is used to generate predictions regarding the effects of three different tasks (Personal Information Exchange, Narrative, and Decision-Making) and three different implementation conditions for each task (unplanned, planned but without detail, detailed planning) on the variables of fluency, complexity, and accuracy. The study reports strong effects of planning on fluency and clear effects also on complexity, with a linear relationship between degree of planning and degree of complexity. However, a more complex relationship was discovered between planning and accuracy, with the most accurate performance produced by the less detailed planners. In addition, interactions were found between task type and planning conditions, such that the effects of planning were greater with the Narrative and Decision-Making tasks than with the Personal Information Exchange task. The results are discussed in terms of an attentional model of learning and performance and highlight the importance of trade-off effects between the goals of complexity and accuracy in the context of the use of limited capacity attentional resources. The study contributes to the development of cognitive models of second language performance and addresses a number of pedagogic issues.