Multiple domains develop simultaneously and interact throughout infancy and early childhood. Although relationships between motor and language skills have been examined cross-sectionally during the first three years of life, little is known regarding the individual factors that influence the development of these domains as well as the relationship between these domains. The present study addressed these knowledge gaps by evaluating the longitudinal trajectory of motor and language skills in a sample of infants (n= 50) ages 1 - 3 years of age, representing a broad range of socioeconomic status (SES). Performance on standardized motor and language assessments were examined with respect to age and sex -- biologically assigned at birth (Specific Aim 1). After accounting for age and sex, socioeconomic factors were examined to determine their influence on each domain (Specific Aim 2). After controlling for age and sex, relationships between motor and language domains were examined (Specific Aim 2). Finally, a mediation analysis was used to determine if the relationships between SES factors and language domains is mediated by motor development (Specific Aim 3). First, it is hypothesized that age is positively related to the motor and language development (separately); it is unclear if sex will influence the development of these skills. Second, SES is positively associated with motor and language skills after accounting for age and sex. Third, it is hypothesized that there is a positive relationship between motor and language skills after accounting for age and sex. Fourth, motor development mediates the relationship between SES and language skills after accounting for age and sex. Overall, the results of this study are relevant to parents, clinicians, and early detection and intervention programs for motor and language skills during the first three years of life. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]