Grounded in a review of existing theories and research on problem solving, the theoretical base and new instrument development efforts discussed in this publication have been sounded against the needs of an innovative, interdisciplinary curriculum project called Unified Science and Mathematics for Elementary Schools (USMES). It is the work on new instrument development for problem solving which is the focus of this document. The report is addressed to those concerned with the evaluation of USMES but also to a wider audience whose concerns may embrace the evaluation of other curricula for elementary schools, research on child development, or theoretical development of models of problem solving. After establishing the need for new instrument development in problem solving, discussing various views of problem solving, and reviewing existing measures of problem solving, this report details the development of a paper-and-pencil Test of Problem Solving Skills (TOPSS) and the development of PROFILES: an interview/observation technique to assess problem solving processes in children. The chapter on TOPSS includes the search for items, pilot testing the instrument, and technical information on item analysis, reliability and validity. The chapter on PROFILES includes information on its rationale, its development, the importance of observer training and monitoring, procedures for its administration, and the development of a scoring protocol. (RC)