Despite the enhanced importance given by science educators and researchers toevaluation in sciences in the past, the search for better and more reliable assessingprocesses is nowadays a research priority.Although the areas of science to be assessed have been generally accepted asincluding knowledge of concepts and facts, process skills, science thinking, problemsolvingskills, abilities reeded to manipulate laboratoty equipment and the dispositionof students to apply scientific knowledge, the impact of assessment on students,teachers and parents has not been overlooked. However, advances in cognitivepsychology, science education and research on assessment on science are callingfor new dimensions, such as complex reasoning, to be researched in the field.Complex reasoning is a competence included as a desirable outcome of Scienceeducation in many Science curricula and is characterized by the following attributes:problem-solving, decision-making and critical and creative thinking. This paperpresents a theoretical framework fon assessment in science involving thedevelopment of competencies through science education and emphasizes complexreasoning. A working model to categorize assessment tasks in complex reasoning isdescribed and some of the main questions to be researched in the une are stated.The need for interdisciplinary work as well as close interaction with other unes ofinvestigation has also been put forward.It is the main objective of this line of nesearch to explore and propose moresystematic assessment processes in science education to trace the evolution ofstudents understandings and achievements mainly in secondary schools.