## TL;DR

Tokenization splits text into units (tokens) that the model processes. Methods: word-level (large vocabulary, OOV problem), character-level (tiny vocabulary, long sequences), subword (balanced: BPE, WordPiece, SentencePiece — standard for modern LLMs). Subword tokenization handles rare and unseen words by decomposing into known subword units.

## Core Explanation

BPE (Byte-Pair Encoding): iteratively merge most frequent adjacent token pairs. WordPiece (BERT): merges tokens that maximize likelihood — difference: BPE merges by frequency, WordPiece by language model probability. SentencePiece (T5, LLaMA): treats input as raw text, language-agnostic (no pre-tokenization). Token vocabulary size: typical LLMs use 30K-250K tokens.

## Further Reading

- [Neural Machine Translation of Rare Words with Subword Units (Sennrich et al., 2016)](https://arxiv.org/abs/1508.07909)