diff --git a/src/block.rs b/src/block.rs
index 3c42e4e27..f98934988 100644
--- a/src/block.rs
+++ b/src/block.rs
@@ -162,4 +162,4 @@ fn open_block() {
 	let mut db = OverlayDB::new_temp();
 	engine.spec().ensure_db_good(&mut db);
 	let b = OpenBlock::new(engine.deref(), db, &genesis_header, vec![genesis_header.hash()]);
-}
\ No newline at end of file
+}
diff --git a/src/engine.rs b/src/engine.rs
index 7fee085e9..7d86aa0a1 100644
--- a/src/engine.rs
+++ b/src/engine.rs
@@ -1,6 +1,7 @@
 use common::*;
 use block::Block;
 use spec::Spec;
+use verification::VerificationError;
 
 /// A consensus mechanism for the chain. Generally either proof-of-work or proof-of-stake-based.
 /// Provides hooks into each of the major parts of block import.
@@ -25,7 +26,7 @@ pub trait Engine {
 	fn evm_schedule(&self, env_info: &EnvInfo) -> EvmSchedule;
 
 	/// Some intrinsic operation parameters; by default they take their value from the `spec()`'s `engine_params`.
-	fn maximum_extra_data_size(&self, _env_info: &EnvInfo) -> usize { decode(&self.spec().engine_params.get("maximumExtraDataSize").unwrap()) }
+	fn maximum_extra_data_size(&self) -> usize { decode(&self.spec().engine_params.get("maximumExtraDataSize").unwrap()) }
 	fn account_start_nonce(&self) -> U256 { decode(&self.spec().engine_params.get("accountStartNonce").unwrap()) }
 
 	/// Block transformation functions, before and after the transactions.
@@ -36,12 +37,12 @@ pub trait Engine {
 	/// `parent` (the parent header) and `block` (the header's full block) may be provided for additional
 	/// checks. Returns either a null `Ok` or a general error detailing the problem with import.
 	// TODO: consider including State in the params.
-	fn verify_block(&self, _header: &Header, _parent: Option<&Header>, _block: Option<&[u8]>) -> Result<(), EthcoreError> { Ok(()) }
+	fn verify_block(&self, _mode: VerificationMode, _header: &Header, _parent: Option<&Header>, _block: Option<&[u8]>) -> Result<(), VerificationError> { Ok(()) }
 
 	/// Additional verification for transactions in blocks.
 	// TODO: Add flags for which bits of the transaction to check.
 	// TODO: consider including State in the params.
-	fn verify_transaction(&self, _t: &Transaction, _header: &Header) -> Result<(), EthcoreError> { Ok(()) }
+	fn verify_transaction(&self, _t: &Transaction, _header: &Header) -> Result<(), VerificationError> { Ok(()) }
 
 	/// Don't forget to call Super::populateFromParent when subclassing & overriding.
 	// TODO: consider including State in the params.
@@ -55,3 +56,11 @@ pub trait Engine {
 
 	// TODO: sealing stuff - though might want to leave this for later.
 }
+
+#[derive(Debug, PartialEq, Eq)]
+pub enum VerificationMode {
+	/// Do a quick and basic verification if possible.
+	Quick,
+	/// Do a full verification.
+	Full
+}
diff --git a/src/ethereum/ethash.rs b/src/ethereum/ethash.rs
index 433a03681..8e26e8f86 100644
--- a/src/ethereum/ethash.rs
+++ b/src/ethereum/ethash.rs
@@ -2,6 +2,7 @@ use common::*;
 use block::*;
 use spec::*;
 use engine::*;
+use verification::*;
 
 /// Engine using Ethash proof-of-work consensus algorithm, suitable for Ethereum
 /// mainnet chains in the Olympic, Frontier and Homestead eras.
@@ -25,6 +26,91 @@ impl Engine for Ethash {
 		let a = block.header().author.clone();
 		block.state_mut().add_balance(&a, &decode(&self.spec().engine_params.get("block_reward").unwrap()));
 	}
+
+	fn verify_block(&self, mode: VerificationMode, header: &Header, parent: Option<&Header>, block: Option<&[u8]>) -> Result<(), VerificationError> { 
+		if mode == VerificationMode::Quick {
+			let min_difficulty = decode(self.spec().engine_params.get("minimumDifficulty").unwrap());
+			if header.difficulty < min_difficulty {
+				return Err(VerificationError::block(
+					BlockVerificationError::InvalidDifficulty { required: min_difficulty, got: header.difficulty }, 
+					block.map(|b| b.to_vec())));
+			}
+			let min_gas_limit = decode(self.spec().engine_params.get("minGasLimit").unwrap());
+			if header.gas_limit < min_gas_limit {
+				return Err(VerificationError::block(
+					BlockVerificationError::InvalidGasLimit { min: min_gas_limit, max: From::from(0), got: header.gas_limit }, 
+					block.map(|b| b.to_vec())));
+			}
+			let len: U256 = From::from(header.extra_data.len());
+			let maximum_extra_data_size: U256 = From::from(self.maximum_extra_data_size());
+			if header.number != From::from(0) && len > maximum_extra_data_size {
+				return Err(VerificationError::block(
+					BlockVerificationError::ExtraDataTooBig { required: maximum_extra_data_size, got: len }, 
+					block.map(|b| b.to_vec())));
+			}
+			match parent {
+				Some(p) => {
+					// Check difficulty is correct given the two timestamps.
+					let expected_difficulty = self.calculate_difficuty(header, p);
+					if header.difficulty != expected_difficulty {
+						return Err(VerificationError::block(
+							BlockVerificationError::InvalidDifficulty { required: expected_difficulty, got: header.difficulty }, 
+							block.map(|b| b.to_vec())));
+					}
+					let gas_limit_divisor = decode(self.spec().engine_params.get("gasLimitBoundDivisor").unwrap());
+					let min_gas = p.gas_limit - p.gas_limit / gas_limit_divisor;
+					let max_gas = p.gas_limit + p.gas_limit / gas_limit_divisor;
+					if header.gas_limit <= min_gas || header.gas_limit >= max_gas {
+						return Err(VerificationError::block(
+							BlockVerificationError::InvalidGasLimit { min: min_gas_limit, max: max_gas, got: header.gas_limit }, 
+							block.map(|b| b.to_vec())));
+					}
+				},
+				None => ()
+			}
+			// TODO: Verify seal
+		}
+		Ok(())
+	}
+
+	fn verify_transaction(&self, _t: &Transaction, _header: &Header) -> Result<(), VerificationError> { Ok(()) }
+}
+
+impl Ethash {
+	fn calculate_difficuty(&self, header: &Header, parent: &Header) -> U256 {
+		const EXP_DIFF_PERIOD: u64 = 100000;
+		if header.number == From::from(0) {
+			panic!("Can't calculate genesis block difficulty");
+		}
+
+		let min_difficulty = decode(self.spec().engine_params.get("minimumDifficulty").unwrap());
+		let difficulty_bound_divisor = decode(self.spec().engine_params.get("difficultyBoundDivisor").unwrap());
+		let duration_limit = decode(self.spec().engine_params.get("durationLimit").unwrap());
+		let frontier_limit = decode(self.spec().engine_params.get("frontierCompatibilityModeLimit").unwrap());
+		let mut target = if header.number < frontier_limit {
+			if header.timestamp >= parent.timestamp + duration_limit {
+				parent.difficulty - (parent.difficulty / difficulty_bound_divisor) 
+			} 
+			else {
+				parent.difficulty + (parent.difficulty / difficulty_bound_divisor)
+			}
+		}
+		else {
+			let diff_inc = (header.timestamp - parent.timestamp) / From::from(10);
+			if diff_inc <= From::from(1) {
+				parent.difficulty + parent.difficulty / From::from(2048) * (U256::from(1) - diff_inc)
+			}
+			else {
+				parent.difficulty - parent.difficulty / From::from(2048) * max(diff_inc - From::from(1), From::from(99))
+			}
+		};
+		target = max(min_difficulty, target);
+		let period = ((parent.number + From::from(1)).as_u64() / EXP_DIFF_PERIOD) as usize;
+		if period > 1 {
+			target = max(min_difficulty, target + (U256::from(1) << (period - 2)));
+		}
+		target
+	}
 }
 
 // TODO: test for on_close_block.
@@ -44,3 +130,4 @@ fn playpen() {
 	let b = OpenBlock::new(engine.deref(), db, &genesis_header, vec![genesis_header.hash()]);
 //	let c = b.close();
 }
+
diff --git a/src/lib.rs b/src/lib.rs
index 2154a7c14..b1c6b0a31 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -102,4 +102,5 @@ pub mod extras;
 pub mod client;
 pub mod sync;
 pub mod block;
+pub mod verification;
 pub mod ethereum;
diff --git a/src/verification.rs b/src/verification.rs
index 95951da53..f4aa4dcb6 100644
--- a/src/verification.rs
+++ b/src/verification.rs
@@ -1,10 +1,35 @@
-use util::uint::*;
-use util::hash::*;
-use util::rlp::*;
-use util::sha3::Hashable;
-use util::triehash::ordered_trie_root;
+use util::*;
 use header::Header;
 use client::BlockNumber;
+use engine::{Engine, VerificationMode};
+use views::BlockView;
+
+#[derive(Debug)]
+pub struct VerificationError {
+	pub block: Option<Bytes>,
+	pub error: VerificationErrorOption,
+}
+
+impl VerificationError {
+	pub fn block(error: BlockVerificationError, block: Option<Bytes>) -> VerificationError {
+		VerificationError {
+			block: block,
+			error: VerificationErrorOption::Block(error),
+		}
+	}
+	pub fn transaction(error: TransactionVerificationError, block: Option<Bytes>) -> VerificationError {
+		VerificationError {
+			block: block,
+			error: VerificationErrorOption::Transaction(error),
+		}
+	}
+}
+
+#[derive(Debug)]
+pub enum VerificationErrorOption {
+	Transaction(TransactionVerificationError),
+	Block(BlockVerificationError),
+}
 
 #[derive(Debug)]
 pub enum TransactionVerificationError {
@@ -18,7 +43,6 @@ pub enum TransactionVerificationError {
 		used: U256,
 		limit: U256
 	},
-	ExtraDataTooBig,
 	InvalidSignature,
 	InvalidTransactionFormat,
 }
@@ -30,8 +54,12 @@ pub enum BlockVerificationError {
 		limit: U256,
 	},
 	InvalidBlockFormat,
+	ExtraDataTooBig {
+		required: U256,
+		got: U256,
+	},
 	InvalidUnclesHash {
-		expected: H256,
+		required: H256,
 		got: H256,
 	},
 	TooManyUncles,
@@ -42,11 +70,18 @@ pub enum BlockVerificationError {
 	InvalidStateRoot,
 	InvalidGasUsed,
 	InvalidTransactionsRoot {
-		expected: H256,
+		required: H256,
 		got: H256,
 	},
-	InvalidDifficulty,
-	InvalidGasLimit,
+	InvalidDifficulty {
+		required: U256,
+		got: U256,
+	},
+	InvalidGasLimit {
+		min: U256,
+		max: U256,
+		got: U256,
+	},
 	InvalidReceiptsStateRoot,
 	InvalidTimestamp,
 	InvalidLogBloom,
@@ -93,17 +128,30 @@ pub fn verify_block_integrity(block: &[u8], transactions_root: &H256, uncles_has
 	let expected_root = ordered_trie_root(tx.iter().map(|r| r.as_raw().to_vec()).collect()); //TODO: get rid of vectors here
 	if &expected_root != transactions_root {
 		return Err(BlockVerificationError::InvalidTransactionsRoot {
-			expected: expected_root.clone(),
+			required: expected_root.clone(),
 			got: transactions_root.clone(),
 		});
 	}
 	let expected_uncles = block.at(2).as_raw().sha3();
 	if &expected_uncles != uncles_hash {
 		return Err(BlockVerificationError::InvalidUnclesHash {
-			expected: expected_uncles.clone(),
+			required: expected_uncles.clone(),
 			got: uncles_hash.clone(),
 		});
 	}
 	Ok(())
 }
 
+pub fn verify_block_basic(bytes: &[u8], parent: &Header, engine: &mut Engine) -> Result<(), BlockVerificationError> {
+	let block = BlockView::new(bytes);
+	let header = block.header();
+	try!(verify_header(&header));
+	try!(verify_parent(&header, parent));
+	try!(verify_block_integrity(bytes, &header.transactions_root, &header.uncles_hash));
+
+	Ok(())
+}
+
+pub fn verify_block_unordered(block: &[u8]) -> Result<(), BlockVerificationError> {
+	Ok(())
+}