This paper analyses the strategies used by pre-service primary school teachers for solving simple addition problems involving negative numbers. The findings reveal six different strategies that depend on the difficulty of the problem and, in particular, on the unknown quantity. We note that students use negative numbers in those problems they find easy and resort to other strategies in more complex problems. Furthermore, the problem-solving strategies reveal lingering difficulties involving negative numbers, such as mistakes with arithmetic rules and a lack of rigour in the mathematical notation used in the operations.