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This doctoral dissertation consists of a series of four manuscripts in which we investigated the influence of the previously used strategy on the subsequent strategy choice (i.e., the so-called perseveration effect). Although sequential effects have already been widely studied in a variety of research domains, they have scarcely been investigated with respect to individuals' strategy choice behaviour. All reported experiments in this dissertation relied on the same experimental task, namely the numerosity judgement task. The goal of this task is to determine various numerosities of coloured cells in a rectangular 5 x 10 grids. Participants can choose between two strategies to solve the different problems: an addition strategy (i.e., adding the coloured cells individually or groupwise) and a subtraction strategy (i.e., adding the empty cells individually or groupwise and subtracting this number from the grid size). The dissertation starts with a general introduction in which we provide a background about multiple strategy use, sequential effects in other research domains, and the numerosity judgement task. In Chapter 1, we report the first experiments that tested this perseveration effect in strategy choices. Therefore, sequences were built in which test items (that were assumed to elicit both strategies) were preceded by addition items or subtraction items that only elicited the addition or the subtraction strategy, respectively. We selected these test items from both a broad (first experiment) and a small numerosity range (second experiment). The results confirmed the hypothesized perseveration effect, that is, participants chose more often for the subtraction strategy after the multiple use of the subtraction strategy than after the multiple use of the addition strategy. However, it was also found that the effect was limited to these numerosities for which both strategies were more or less equally applicable. The goal of the following experiment, reported in Chapter 2, was to replicate the findings observed in the first two experiments with a different research paradigm. In this experiment, we manipulated the presentation order in which the different numerosities were presented (i.e., an ascending, a descending, and a random order). Also with this research paradigm, we were able to show the perseveration effect. Common to the experiments of Chapter 1 and Chapter 2 is that they all showed the perseveration effect after a repeated use of the previous strategy. However, it is also important to know whether the perseveration effect would already show up after a single previous strategy application, and, if so, whether the strength of the effect would be different after a single versus a repeated application of a strategy. Therefore, we conducted an experiment (Chapter 3) with two different conditions, namely a repeat condition (i.e., the test item was presented after five strategy applications) and a single condition (i.e., the test item was presented after a single strategy application). This experiment revealed that a single previous strategy application was sufficient to elicit the perseveration effect. Moreover, the strength of the perseveration effect did not increase as a function of the number of previous strategy applications. Interestingly, an additional cluster analysis showed large individual differences in participants’ strategy choices. Three groups could be distinguished: a group showing the perseveration effect (i.e., a group who used the addition strategy most often after addition items, and the subtraction strategy most often after subtraction items), and two groups who did not show the perseveration effect but who had a strong preference for either the addition strategy or the subtraction strategy (irrespective of the preceding strategy).This latter finding led to a follow-up experiment (Chapter 4) in which we tried to find out how these individual differences in the perseveration effect could be explained. Five different subject characteristics (i.e., inhibition, switching, updating, arithmetic skills, and subtraction self-efficacy beliefs) were tested to further unravel this finding. The results showed that two of these subject characteristics could at least explain some of the individual differences, namely, inhibition and subtraction self-efficacy beliefs.The dissertation ends with Chapter 5, in which we provide a general discussion of some mechanisms that may underlie the observed perseveration effect, address some limitations of the reported studies of this dissertation, discuss some educational implications, and we will also give some directions for further research.