In their series of professional books for teachers, Young Mathematicians at Work , Catherine Twomey Fosnot and Maarten Dolk described Mathematics in the City, an innovative project where teachers helped young children construct a deep understanding of number and operation in a math-workshop environment. Now, they and two colleagues from the project have developed a flexible, video-based, digital context for inquiry into the teaching and learning of mathematics that will change how professional development is conducted. Designed for you, the workshop leader or college instructor, the Working with the Array Resource Package enables your in- or pre-service teachers to not only watch but interact with video that depicts classroom teachers as they listen to, question, and interpret students’ thinking; develop connections between mathematical ideas and strategies; and, ultimately, develop vibrant mathematical communities in their classrooms. The Resource Package includes three valuable components: a completely interactive CD-ROM, where your workshop participants can explore - independently or under your guidance; videos of instruction and assessment; and, sample children’s work over time to analyze development. They can take and save notes on what they see, capture specific frames or footage, and then email their captured video clips and notes to other members of your professional development workshop. The context of the classroom will be at the fingertips of your participants for exploration. A Professional Development Overview Manual provides general advice on how you can use the CD-ROM for staff development. It also contains a Facilitator’s Guide whose field-tested content is specific to the CD-ROM and includes helpful suggestions for using video clips and student examples on the CD to design rich professional development experiences; sample dialogue to help you anticipate what your participants might say; tips for facilitating discussions among teachers; and descriptions of the mathematical ideas being explored. In Working with the Array , your workshop participants will observe fourth graders as they construct some of the big ideas related to surface area and volume, making connections between multiplication’s commutative and associative properties and uncovering landmark division strategies like halving and factoring. By studying the use of carefully crafted problems designed both to generate a range of solution strategies and to highlight the power of arrays and other models for multiplication and division, teachers will discover what a valuable tool real-life contexts are for building a solid foundation.