Debris flows can grow greatly in size and hazardous potential by eroding bed and bank materials. However, erosion mechanisms are poorly understood because debris flows are complex hybrids between a fluid flow and a moving mass of colliding particles, bed erodibility varies between events, and field measurements are hard to obtain. Here, we identify the key controls on debris‐flow erosion based on a field data set that combines information on flow properties, bed conditions, and bed and bank erosion. We show that flow conditions and bed wetness jointly control debris‐flow erosion. Flow conditions describing the cumulative forces exerted at the bed during an event best explain erosion. Shear forces and particle‐impact forces are strongly correlated and act in conjunction in the erosion process. A shear‐stress approach accounting for bed erodibility may therefore be applicable for modeling and predicting debris‐flow erosion. This work provides a foundation for developing effective debris‐flow erosion models. Plain Language Summary Debris flows are water‐laden masses of soil and rock, which are common geological hazards in mountainous regions worldwide. They can grow greatly in size and hazardous potential by eroding bed and bank materials. Limited understanding of these erosion processes, however, hampers effective hazard assessment and mitigation. Improving our understanding of erosion is challenging because debris flows are complex hybrids between a fluid flow and a moving mass of colliding particles, bed erodibility varies between events, and field measurements are hard to obtain. Here, we identify the key controls on debris‐flow erosion based on a field data set that combines information on flow properties, bed conditions, and bed and bank erosion. We show that flow properties and bed wetness jointly control debris‐flow erosion. Flow conditions that describe the cumulative forces exerted at the bed during an event best explain erosion. Shear forces and particle‐impact forces are strongly correlated and act in conjunction in the erosion process. A shear‐stress approach accounting for bed erodibility may therefore be applicable for modeling and predicting debris‐flow erosion. This work provides a foundation for developing effective debris‐flow erosion models. Key Points Flow conditions and bed wetness jointly control debris‐flow erosion and deposition Shear forces and particle‐impact forces are strongly correlated and together determine erosion A shear‐stress approach accounting for bed erodibility may be applicable for modeling debris‐flow erosion