In their Young Mathematicians at Work series, 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 operations in a math workshop environment. Now they and two colleagues from the project have developed a 21st-century approach that provides pre- and inservice teachers with an interactive, video-based, digital context for inquiry into the teaching and learning of mathematics. Designed for workshop participants and preservice teachers, the Exploring Ages CD-ROM offers users a multi-media learning environment for professional development on topics ranging from instruction to assessment, and that stimulates action, reflection, and discussion with dynamic video clips, children’s work samples, and interviews with students and teachers. In Exploring Ages, students explore addition and subtraction through the lens of ages, building new understandings and strategies and learning concepts like removal, missing addends, and constant difference as they solve problems that reflect their natural curiosity. Users also study teachers at work: designing rich contexts for problem-solving, drawing connections between mathematical ideas and strategies, and developing a vibrant mathematical community.System Requirements for CD-ROMWindows/PC Pentium II Processor 266Mhz (or higher) Windows 98 (or higher) 64 MB RAM (more recommended) SVGA Color Display (or better) 4X CD-ROM Drive (or faster) Sound Card 16-bit FlashTM Player and Acrobat Reader Quicktime 6.0 (or higher) MacIntosh PowerPC Processor G3/233Mhz (or higher) System 9.2 (orhigher) 64 MB RAM (more recommended) SVGA Color Display (or better) 4X CD-ROM Drive (or faster) Sound Card 16-bit FlashTM Player and Acrobat Reader Quicktime 6.0 (or higher)