diff --git a/src/chainparams.h b/src/chainparams.h
index 6b1f813af..dd029b9d5 100644
--- a/src/chainparams.h
+++ b/src/chainparams.h
@@ -25,6 +25,12 @@ struct CCheckpointData {
     MapCheckpoints mapCheckpoints;
 };
 
+/**
+ * Holds various statistics on transactions within a chain. Used to estimate
+ * verification progress during chain sync.
+ *
+ * See also: CChainParams::TxData, GuessVerificationProgress.
+ */
 struct ChainTxData {
     int64_t nTime;
     int64_t nTxCount;
diff --git a/src/validation.cpp b/src/validation.cpp
index 3a03d206c..ab44e73d7 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -154,6 +154,10 @@ public:
 
     bool ActivateBestChain(CValidationState &state, const CChainParams& chainparams, std::shared_ptr<const CBlock> pblock);
 
+    /**
+     * If a block header hasn't already been seen, call CheckBlockHeader on it, ensure
+     * that it doesn't descend from an invalid block, and then add it to mapBlockIndex.
+     */
     bool AcceptBlockHeader(const CBlockHeader& block, CValidationState& state, const CChainParams& chainparams, CBlockIndex** ppindex);
     bool AcceptBlock(const std::shared_ptr<const CBlock>& pblock, CValidationState& state, const CChainParams& chainparams, CBlockIndex** ppindex, bool fRequested, const CDiskBlockPos* dbp, bool* fNewBlock);
 
@@ -185,6 +189,11 @@ private:
     CBlockIndex* AddToBlockIndex(const CBlockHeader& block);
     /** Create a new block index entry for a given block hash */
     CBlockIndex * InsertBlockIndex(const uint256& hash);
+    /**
+     * Make various assertions about the state of the block index.
+     *
+     * By default this only executes fully when using the Regtest chain; see: fCheckBlockIndex.
+     */
     void CheckBlockIndex(const Consensus::Params& consensusParams);
 
     void InvalidBlockFound(CBlockIndex *pindex, const CValidationState &state);
@@ -2654,6 +2663,10 @@ static void NotifyHeaderTip() {
  * Make the best chain active, in multiple steps. The result is either failure
  * or an activated best chain. pblock is either nullptr or a pointer to a block
  * that is already loaded (to avoid loading it again from disk).
+ *
+ * ActivateBestChain is split into steps (see ActivateBestChainStep) so that
+ * we avoid holding cs_main for an extended period of time; the length of this
+ * call may be quite long during reindexing or a substantial reorg.
  */
 bool CChainState::ActivateBestChain(CValidationState &state, const CChainParams& chainparams, std::shared_ptr<const CBlock> pblock) {
     // Note that while we're often called here from ProcessNewBlock, this is
@@ -3357,6 +3370,9 @@ bool CChainState::AcceptBlockHeader(const CBlockHeader& block, CValidationState&
         if (!ContextualCheckBlockHeader(block, state, chainparams, pindexPrev, GetAdjustedTime()))
             return error("%s: Consensus::ContextualCheckBlockHeader: %s, %s", __func__, hash.ToString(), FormatStateMessage(state));
 
+        // If the previous block index isn't valid, determine if it descends from any block which
+        // has been found invalid (g_failed_blocks), then mark pindexPrev and any blocks
+        // between them as failed.
         if (!pindexPrev->IsValid(BLOCK_VALID_SCRIPTS)) {
             for (const CBlockIndex* failedit : m_failed_blocks) {
                 if (pindexPrev->GetAncestor(failedit->nHeight) == failedit) {