We present a new concept of biclique as a tool for preimage attacks, which employs many powerful techniques from differential cryptanalysis of block ciphers and hash functions.The new tool has proved to be widely applicable by inspiring many authors to publish new results of the full versions of AES, KASUMI, IDEA, and Square. In this paper, we show how our concept leads to the first cryptanalysis of the round-reduced Skein hash function, and describe an attack on the SHA-2 hash function with more rounds than before.