Cross-laminated timber (CLT) is a building component used in walls, floors, roofs, or beams in a building. The advantages of using CLT as a building component are, among others, its high load-carrying capability and the possibility of pre-fabrication. The in-plane shear properties of a CLT panel are the in-plane shear modulus and in-plane shear load carrying capacity. This thesis is solely about the in-plane shear modulus and is intended to increase the understanding about the in-plane shear modulus of CLT panels. The in-plane shear modulus is important and there is a need to better understand and estimate its value. The objective of this work was to contribute to the need of finding a suitable test method to measure the in-plane shear modulus of CLT panels and to find factors affecting the in-plane shear modulus.  Three different methods: the picture frame test, the diagonal compression test and the diaphragm shear test, were used in practice and compared to a theoretical test method, the pure shear test. The three methods were compared by conducting experimental tests and by simulating the methods using finite element (FE). Based on the FE simulations, an equation to calculate the shear modulus was created for each test method.  Results from the FE analyses showed that the picture frame test gave results similar to the theoretical pure shear test models. The reason for this result was that the picture frame test is a biaxial testing method. The diagonal compression test and the diaphragm shear test are uniaxial test method. It was also concluded that the picture frame test has a pure shear state in the measured region. The mean error for the in-plane shear modulus equations was estimated, by comparing results from practical testing and FE simulations, to be -2.5%, +12.6% and +11.8% for the picture frame test, diagonal compression test and diaphragm shear test, respectively. The diagonal compression test was the preferred method to use with respect to its simplicity.The factors having an impact on the in-plane shear modulus were found by comparing multiple FE simulations. The results showed that it is possible to increase the in-plane shear modulus by: increasing the odd numbered layers width-to-thickness ratio; decreasing the odd layers thickness ratio of the CLT panels thickness; increasing the number of layers; reducing the gaps between boards; and using alternative main laminate directions.