Language skills of the pre-school children are an important prerequisite for the future success at school. While monolingual children usually develop age-appropriate language skills prior to the school entrance, bilingual children may face a variety of different problems. In this empirical study, the language skills of 8 German monolingual and 8 German-Russian bilingual children between 4 and 6 years of ages are examined. The present paper focuses upon the level of the language development of monolingual children in comparison with the bilingual ones as well as upon the mutual relationship between the two languages in the speech of bilingual children. The study aims at examining the language competence of children in the morpho-syntactic area and in the area of static localisation. The analysis was carried out on the basis of linguistic data collected by means of the elicitation methods analyzing the word order, forms and the position of the verb, subject-verb-congruence, the connection between sentences and irregular verbs. The results suggest that the level of the mono- and bilingual children is comparable. This conclusion is also supported by the analysis of the vocabulary which was examined as well. With regard to the localisation, both similarities and differences were observed. It should be emphasized that the relation AN caused considerable problems for bilingual children. The substitution rate of prepositions was substantially higher in these children compared to the monolingual ones. The comparison of the language development of the bilingual children showed clear differences in the command of the German and Russian languages. These differences range from a relatively balanced bilingualism over a strong dominance of German to an almost passive command of the mother tongue. The study showed that a clear separation of the two languages has a positive effect on the bilingual language development. The lack of such clear separation leads to deficits in the first language development, resulting in mixing up the languages and transferring elements of the second language into the first one.