The present paper examines glottal stops and the glottalisation of word-initial vowels in Polish and German. The presence of glottal marking is studied depending on speech style (‘speech’ vs. ‘dialogue’), prominence, phrasal position, speech rate, word type, preceding segment, and following vowel height. A question is also posed about the extent to which glottal marking might be dependent on the rhythmic structure of a given language or style. We analyzed recordings of 18 Polish and German speakers. The results point to significant differences between the two languages. In German, glottal marking occurs significantly more often (63.4%) than in Polish (45%). Whereas in both languages (and both styles) the majority of prominent vowels are more often glottally marked than non-prominent vowels, in German word-initial non-prominent syllables are also marked relatively often. Regarding phrase position, glottal marking occurs significantly more often at the phrase-initial position compared to phrase-medial position in Polish, while no such difference has been found in German. In addition, it is shown that in both languages glottal marking is strongly dependent on the tongue height of the marked vowel: low vowels are more frequently glottalised than non-low vowels. Finally, glottal marking in Polish is more likely to occur when rhythmic variability shifts towards the ‘indeterminate’, strengthening the hypothesis that glottal marking facilitates perceptual grouping.