One in nine people between the ages of 18 and 64 in the US, and every second foreign-born person in this age bracket, speaks Spanish at home. And whereas around 80 percent of adult immigrants in the US from non-English speaking countries other than Mexico are proficient in English, only about 50 percent of adult immigrants from Mexico are proficient. The use of a language other than English at home, and proficiency in English, are both analyzed in this paper using economic models and data from the 2000 US Census. The results demonstrate the importance of immigrants' educational attainment, their age at migration and years spent in the US to their language skills. The immigrants' mother tongue is also shown to affect their English proficiency; immigrants with a mother tongue more distant from English being less likely to be proficient. Finally, immigrants living in ethnic enclaves have lesser proficiency in English than immigrants who live in predominately English-speaking areas of the US. The results for females are generally very similar to those for males, the findings from an ordered probit approach to estimation are similar to the findings from a binary probit model, and the conclusions drawn from the analyses mirror those in studies based on the 1980 and 1990 US Censuses. Thus, the model of language skills presented appears to be remarkably robust across time and between the genders.