This book was created because my teachers reported a lack of materials to help build students’ Missing Number fluency. In response to my teacher’s needs, I created this book to support my teachers, but mostly to help my students build mathematical fluency to help them be better prepared for life in the 21st Century.There are missing number fluency builders for skip counting by 1, 2, 3, 4, 5 and 10. The numbers go up to 99. This book begins with five fluency building sheets forward counting by one’s. Then five fluency building sheets forward counting by two’s, etc. After the forward fluency building sheets counting by 10’s, there are fifteen fluency building sheets forward counting by mixed numbers (1,2,3,4,5 or 10). The next 10 fluency building sheets skip count forward and backward with mixed numbers (1,2,3,4,5 or 10). The final 15 activities are application driven and should not be a part of a timed assessment. These are designed to deepen children’s thinking processes through application of the concept. An answer key is provided for the teacher’s convenience following each fluency builder and application page. A hundreds board is located on the last page for children needing differentiated support. Approximately fifty percent of the missing number blanks are in the first position. This is by design, because a teacher told me students need the most help when the leading number is missing. A third grade teacher also used this book and reported an increase in her students’ multiplication fluency. A writing prompt is located at the bottom of all the fluency builders to be completed on the back. The writing prompt is not a part of the timed assessment, but is pivotal for the development of children’s thinking processes.