diff --git a/core/types/block.go b/core/types/block.go
index 9803d344a0..24472a2830 100644
--- a/core/types/block.go
+++ b/core/types/block.go
@@ -178,7 +178,23 @@ type Body struct {
 	Uncles       []*Header
 }
 
-// Block represents an entire block in the Ethereum blockchain.
+// Block represents an Ethereum block.
+//
+// Note the Block type tries to be 'immutable', and contains certain caches that rely
+// on that. The rules around block immutability are as follows:
+//
+//   - We copy all data when the block is constructed. This makes references held inside
+//     the block independent of whatever value was passed in.
+//
+//   - We copy all header data on access. This is because any change to the header would mess
+//     up the cached hash and size values in the block. Calling code is expected to take
+//     advantage of this to avoid over-allocating!
+//
+//   - When new body data is attached to the block, a shallow copy of the block is returned.
+//     This ensures block modifications are race-free.
+//
+//   - We do not copy body data on access because it does not affect the caches, and also
+//     because it would be too expensive.
 type Block struct {
 	header       *Header
 	uncles       []*Header
@@ -270,8 +286,7 @@ func NewBlock(header *Header, body *Body, receipts []*Receipt, hasher TrieHasher
 	return b
 }
 
-// CopyHeader creates a deep copy of a block header to prevent side effects from
-// modifying a header variable.
+// CopyHeader creates a deep copy of a block header.
 func CopyHeader(h *Header) *Header {
 	cpy := *h
 	if cpy.Difficulty = new(big.Int); h.Difficulty != nil {
@@ -302,7 +317,7 @@ func CopyHeader(h *Header) *Header {
 	return &cpy
 }
 
-// DecodeRLP decodes the Ethereum
+// DecodeRLP decodes a block from RLP.
 func (b *Block) DecodeRLP(s *rlp.Stream) error {
 	var eb extblock
 	_, size, _ := s.Kind()
@@ -314,7 +329,7 @@ func (b *Block) DecodeRLP(s *rlp.Stream) error {
 	return nil
 }
 
-// EncodeRLP serializes b into the Ethereum RLP block format.
+// EncodeRLP serializes a block as RLP.
 func (b *Block) EncodeRLP(w io.Writer) error {
 	return rlp.Encode(w, extblock{
 		Header: b.header,
@@ -333,7 +348,14 @@ func (b *StorageBlock) DecodeRLP(s *rlp.Stream) error {
 	return nil
 }
 
-// TODO: copies
+// Body returns the non-header content of the block.
+// Note the returned data is not an independent copy.
+func (b *Block) Body() *Body {
+	return &Body{b.transactions, b.uncles}
+}
+
+// Accessors for body data. These do not return a copy because the content
+// of the body slices does not affect the cached hash/size in block.
 
 func (b *Block) Uncles() []*Header          { return b.uncles }
 func (b *Block) Transactions() Transactions { return b.transactions }
@@ -347,6 +369,13 @@ func (b *Block) Transaction(hash common.Hash) *Transaction {
 	return nil
 }
 
+// Header returns the block header (as a copy).
+func (b *Block) Header() *Header {
+	return CopyHeader(b.header)
+}
+
+// Header value accessors. These do copy!
+
 func (b *Block) Number() *big.Int     { return new(big.Int).Set(b.header.Number) }
 func (b *Block) GasLimit() uint64     { return b.header.GasLimit }
 func (b *Block) GasUsed() uint64      { return b.header.GasUsed }
@@ -374,11 +403,6 @@ func (b *Block) BaseFee() *big.Int {
 	return new(big.Int).Set(b.header.BaseFee)
 }
 
-func (b *Block) Header() *Header { return CopyHeader(b.header) }
-
-// Body returns the non-header content of the block.
-func (b *Block) Body() *Body { return &Body{b.transactions, b.uncles} }
-
 func (b *Block) HashNoNonce() common.Hash {
 	return b.header.HashNoNonce()
 }
@@ -419,10 +443,8 @@ func NewBlockWithHeader(header *Header) *Block {
 // WithSeal returns a new block with the data from b but the header replaced with
 // the sealed one.
 func (b *Block) WithSeal(header *Header) *Block {
-	cpy := *header
-
 	return &Block{
-		header:       &cpy,
+		header:       CopyHeader(header),
 		transactions: b.transactions,
 		uncles:       b.uncles,
 	}