A geologist is a contributor to the science of geology. Geologists study the physical structure and processes of the Earth. Their undergraduate training typically includes significant coursework in chemistry, physics, mathematics and possibly biology, in addition to classes offered through the geology department; volcanology, hydrology, and rock and mineral formation are among the many areas of study. Most geologists also need skills in GIS and other mapping techniques. Geology students may spend summers living and working under field conditions with faculty members. Geology courses are also highly valuable to students of geography, engineering, chemistry, urban planning, archaeology, environmental studies, and other fields.

Professional geologists work for a wide range of government agencies, private firms, and non-profit and academic institutions. Local, state, and national governments hire geologists to help plan and evaluate excavations, construction sites, environmental remediation projects, and natural disaster preparedness, as well as to investigate natural resources. An engineering geologist (a geologist trained, experienced and certified in the field of engineering geology) are called upon to investigate geologic hazards and geologic constraints for the planning, design and construction of public and private engineering projects, forensic and post-mortem studies, environmental impact analysis and other purposes. Petroleum and mining companies and large-scale land developers use geologist's and engineering geologist's skills to help them locate oil and minerals, adapt to local features such as karst deposits or the risk of earthquakes, and comply with environmental regulations. Geologists in academia usually hold an advanced degree in a specialized area within the discipline.

See also: List of geologists