bitsandbytes is a production-grade Python/C++ extension library that enables k-bit quantization for PyTorch. Its actual function is to reduce the memory footprint of large language model training and inference by providing 8-bit blockwise quantized optimizers (Adam, AdamW, LAMB, Lion, AdEMAMix, etc.), 4-bit NF4/FP4 quantized linear layers used by QLoRA, and LLM.int8()-style mixed-precision matmul with outlier separation. It is an application library rather than a data research repository; the research algorithms it implements are packaged as reusable, production-oriented components exposed to downstream libraries such as Hugging Face Transformers, PEFT, Accelerate, TGI, and vLLM. Users interact through quantized nn modules, functional quantize/dequantize primitives, optimizer classes, and autograd matmul functions.