The purpose of the dissertation is to study, describe and analyses which comparative measurement activities preschool children construct and use, and how they solve problems and communicate when they use the comparative measurement activities whilst playing with Lego (“the Lego play”). The measurement activities chosen are length/height, quantity and time. The empirical material is based on data from two preschool classes with children aged 2 – 5 year, It was collected through participant observation (video captures) of the children’s Lego play. The theoretical starting points in this study are grounded in childhood sociology and the sociocultural perspective. The study assumes the childhood sociology perspective by viewing the children as competent and active in creating meaning as well as controlling and influencing their own and others’ social environment. The sociocultural perspective gives prominence to development and learning, and its related tools and concepts are used to analyses the results of the study. That is, the Lego play is studied in a social context from the child’s perspective, and the sociocultural perspective describes and analyses the child’s use of mathematics and the acquisition of knowledge in the Lego play in a sociocultural context. The results show that children measuring length/height and quantity explored a store of measurement tools in order to make comparisons, and adapted these to the context in question. These were own body, other body, artefacts, numbers and counting. The measurements were used individually and with others, and the solving of the own or shared problems constituted a large share of the time spent constructing models during Lego play. By contrast, the time concept was used mainly as a tool when the children played with their finished Lego models. Thus, a time perspective was added to the child’s finished model, which inspired thoughts and reflections about time used in the Lego play. The children used the time concepts of the present, the past and the future, and also considered the concept of velocity in the context of the timescale. The children’s communication had a large impact on the Lego play, and they expressed their ideas verbally, physically and through action. The children’s use of mathematics was prominent and meaningful during the Lego play.