The aim of the master's thesis was to develop and optimize the extraction procedures by using vortex, ultrasound-assisted extraction and accelerated solvent extraction (ASE) for proanthocyanidins and flavonoid quercitrin from leaves of the invasive alien plant species Japanese knotweed (Fallopia japonica Houtt.), which is a rich source of proanthocyanidins and flavonoids. The extraction efficiency was tested by using two high-performance thin-layer chromatographic (HPTLC) densitometric methods on a silica gel stationary phase. Densitometry for proanthocyanidins was performed at λ = 624 nm after post-chromatographic derivatization with 4-dimethylaminocinnamaldehyde reagent, and at λ = 480 nm for quercitrin after post-chromatographic derivatization with 2-aminoethyl diphenylborinate and polyethylene glycol 4000 reagents. For all extraction techniques, the same ratio of solid material and extraction solvent (water, methanol, ethanol, acetone, ethyl acetate and their mixtures) was used. For ASE extraction in addition to solvent optimization, temperature, number of cycles in static extraction, flow rate in dynamic extraction, and extraction cell volume were also optimised. The type of solvent and temperature had the greatest impact on extraction efficiency. For proanthocyanidins, the best results were obtained with ASE extraction using methanol at 140 °C while for quercitrin the best results were obtained with 30 min ultrasound assisted extraction using 70% aqueous solution of acetone. Extract of Japanese knotweed leaves, which was obtained with developed extraction procedure for flavonoids, was used for HPTLC-MS/MS analysis. In addition to flavonoids (flavonols and flavones) other bioactive compounds from the groups of anthraquinones, phenolic and carboxylic acids, stilbenes, and naphthalenes were also determined in Japanese knotweed leaves.