This paper will share the author's experiences working with a large and diverse school district to design curriculum-embedded, performance-based assessments (PBAs) that measure 21st century skills in K-5 students. These assessments are designed to integrate seamlessly with classroom instruction; measure skills such as critical thinking, creativity, and collaboration in a developmentally-appropriate way; and provide information useful for supporting classroom teaching and learning. I begin by describing the context for the partnership. Next, I outline general principles that support the construction of the PBAs and explain the task development process. I also explore challenges in the development and validation of such assessments. Finally, I close the paper by speculating about future trends in PBA.