In vitro experimentation evolution studies have shown that Zymoseptoria tritici can easily develop resistance to different succinate dehydrogenase (Sdh) inhibitor fungicides upon UV exposure. A range of mutations (33) resulting in amino acid substitutions at 21 different positions within SdhB, C and D were identified. Different levels of SDHI sensitivity were linked with different mutations but high insensitivity levels were measured for B-H267L, C-S83G, C-N86K and C-H152R. The first SDHI-insensitive field strains were detected in the UK in 2015, four years after high usage of SDHIs to Septoria leaf blotch control. Strains with C-H152R showed the highest level of in vitro insensitivity to several SDHIs (e.g. boscalid, isopyrazam, bixafen, fluxapyroxad and benzovindiflupyr) and could not be controlled in planta. However, although C-H152R is accumulating after SDHI-based treatments in the field during the season, the frequency remains low at the start of the following season, indicating a fitness penalty that was also reported in experimentation evolution studies. A non-target site resistance mechanism, present as standing variation in field populations, was found to affect only fluopyram and isofetamid. Up to 2018, 22 different Sdh mutations have been reported in UK strains. Mutations resulting in B-H267L and C-N86K were for the first time detected in 2018, as well as strains carrying Sdh mutations in combination with efflux pump overexpression and further monitoring is required to follow the spread of these strains and impact on disease control.