Colluvial deposits are heterogeneous and unconsolidated clastic sediments accumulated at the base of hills or mountain slopes. In recent years, these sediments have received attention in environmental and geological research, as they record valuable data about paleo-tectonic activities, paleoclimate, and weathering conditions in subaerial slopes. This chapter reviews and discusses concepts of colluvial deposits in geosciences and describes lithofacies, depositional processes, models, and practical implications. A combination of field observations, laboratory work, and geotechnical investigations allows for a better understanding of slope processes responsible for colluvial accumulation. These deposits accumulate in various climatic conditions, being transported and deposited mainly by gravity (i.e., mass wasting and mass flow). The main lithofacies are rock falls and debris-flow deposits. Subordinate facies include calcrete/paleosol horizons and water-flow deposits (slope wash). While mass wasting occurs in tectonically unstable periods, soil horizons may develop during periods of tectonic quiescence.