Monitoring the occurrence and co-occurrence of mycotoxins, toxic secondary metabolites of toxigenic fungi, is necessary to determine the safety of the raw material and/or products. Within this doctoral dissertation, a multimycotoxin LC-MS/MS method for the simultaneous determination of 11 regulated mycotoxins in cereals and products (AFB1, AFB2, AFG1, AFG2, DON, FB1, FB2, ZEA, T-2, HT -2 and OTA) was developed. A simple sample preparation was based on solid-liquid extraction using solvent mixture (acetonitrile/water/formic acid, 79/20/1, v/v/v) and dilution of crude extract (acetonitrile/water/formic acid, 20/79/1, v/v/v) before analysis. The fitness of the method for the purpose was proven through the validation of the method, where the tested features of the method met the legally set acceptability criteria. The method was used to analyze 209 cereal samples (maize, wheat, barley, oats, rye and triticale) of 2016 and 2017 harvest from all Croatian counties. The most common metabolites found in cereals in both years were Fusarium mycotoxins, primarily DON and HT-2. A total of 17 samples did not comply with the highest permitted values for food, two of which did not meet the recommended values for animal feed. Out of all investigated samples of 2016 and 2017 harvest, 50.0% and 33.7% of samples respectively, were contaminated with two or more regulated mycotoxins (most frequent in 2016 DON, FB1, FB2, ZEA, T-2 and HT-2 in corn, in 2017 DON and HT-2 in wheat and FB1 and FB2 corn). The most contaminated cereal was maize in both years, with FB1 being the most prevalent mycotoxin. Moderate temperatures with periods of high air humidity favored DON accumulation in wheat samples instead of other Fusarium mycotoxins, while similar conditions favored maize contamination with FUM.