This chapter reports the results of a spontaneous speech study on the interaction between Case and Number in child Russian. Following Hoekstra & Hyams (1995, 1996) and Müller (1994), we hypothesize that the Number head is initially underspecified in child grammars and represents [+singular] only. Our results support this hypothesis, exhibiting almost flawless Case licensing in the singular, but failure to use plural Cased forms correctly between the ages 1;8–2;0 and to a lesser extent between the ages 2;1–2;6. Our results also support the Full Clause or Full Competence Hypothesis (Hyams 1992; Wexler 1992; Poeppel & Wexler 1993), according to which functional categories are present in child grammar from the beginning.