The study found rational preferences for both spatial (place of crossing) and temporal (time of crossing), based on the opportunities and risks presented by the physical and human environments. The busiest months for all types of illegal crossing were January to May, with March being the most active month and December the least active. Most of the illegal border crossings occurred from Thursday to Sunday, with Saturday being the busiest day and Monday the least busy. The pattern was more varied and cyclical for specifically criminal entries, notably drug trafficking; Wednesday was the busiest day and Sunday the least busy. The two peak activity times for all illegal entries were from 10:00 am to 2:00 pm and from 8:00 pm to 10:00 pm. Places of entry tended to correlate with ease of crossing flowing streams; existence of pathways; placement patterns of Border Patrol personnel; and proximity to various human settlement features, including urban areas and city streets. This shows the importance of Mexican settlements as set-up locations for both all illegal entry and criminal entry crossings. The former are also influenced by the location of U.S. border settlements. According to U.S. Border Patrol agents, vegetation may play a role in providing cover where illegal migrants choose to cross the border. The study used a geographic information system to combine illegal entry spatial data with border region information layers in order to analyze their relationship. The dependent variable was the location of illegal border crossings across the Texas-Mexico border. The independent variables were physical and human geographic features. 7 tables, 21 figures, and 29 references