A method and system for allocating and freeing memory is provided. A requesting program running on a computer system allocates a logically contiguous block of memory (a "heap") for the temporary storage of data structures. A Heap Manager, provided by the present invention, manages the heap in response to requests from the requesting program. The Heap Manager logically divides the heap into segments and maintains a free list for each segment. The free list is a list of the free blocks contained in a segment. The heap has an associated free block array and an associated size tree. The free block array contains entries, each entry pointing to a free list. Each leaf node of the size tree corresponds to entries in the free block array. A value contained in each leaf node indicates the size of the largest free block in the segments corresponding to the entries. The Heap Manager selects a free block for allocation to the requesting program by searching to size tree and the free block array and selecting segment that contains a free block that will satisfy the request, and then searching the free list corresponding to the selected segment to locate the smallest free block that satisfies the request.