At the outbreak of World War II, the squadron was equipped with Blenheim IVs and was involved in attacks against enemy barge concentrations in the Channel Ports. In mid-1941, No. 101 Sqn became part of Bomber Command's medium-bomber force with Wellingtons and flew many night sorties against Germany and Italy. These aircraft were replaced barely 18 months later by the Lancaster and became specialised in airborne radar jamming to disrupt German night-fighters interceptions.

On the declaration of war with Germany 101 Squadron initially deployed to its dispersal airfield, RAF Brize Norton, but after a few days it returned to West Raynham. 101 Squadron's Blenheims lacked the latest modifications and the unit spent the next few months as a training and conversion unit for new Blenheim crews in 2 Group. With the fall of France 101 Squadron became operational, but its officer commanding, Wg Cdr J H Hargroves, and his crew were lost on its first bombing mission on 5th July 1940. During the Battle of Britain 101 Squadron Blenhiems carried out attacks on invasion barge concentrations in the French Channel ports and attacked enemy bomber airfields. Throughout the winter of 1940/41 101 Squadron carried out anti-shipping strikes and attacked German and French ports, including Brest where the German battlecruisers Scharnhorst and Gneisenau were based. Another OC 101 Squadron, Wg Cdr D Addenbrooke, was lost on one of these attacks on 3 April, just 3 days after taking command.
 

In April 1941 the Squadron re-equipped with the Vickers Wellington and redeployed to RAF Oakington to join No 3 Group. On the 24th July 101 Squadron lost its first Wellington on another daylight raid against Brest. Early in 1942 101 Squadron carried out trials with the "Gee" navigation aid, which went some way to improve bombing accuracy. On the 30th May 1942 ten 101 Squadron Wellingtons took part in the first 1,000 bomber raid on Cologne, but losses began to mount and between July and September the Squadron lost 20 Wellingtons with 86 aircrew killed.
 

In September 101 Squadron moved to RAF Holme-on-Spalding-Moor and re-equipped to become the first operational Avro Lancaster squadron in No 1 Group. In January 1943 Wg Cdr D A Riddick (who had impressed the Hendon crowds with his Overstrand aerobatics) took command and led the Squadron through the Battles of the Ruhr. On 15 June 1943 101 Squadron moved to its final wartime base, RAF Ludford Magna. From here its Lancaster crews participated in the battle of Hamburg and the raid on the secret German rocket site at Peenemunde. In September 1943 101 Squadron crews began to fly specially modified Lancasters fitted with top secret AIRBORNE CIGAR or ABC radio jamming equipment. An additional "Special Operator" joined each 101 Squadron crew to monitor this equipment. During the winter of 1943/4 No 101 Squadron crews fought in the Battle of Berlin but suffered high casualties. On the 31st March 1944, during the Nuremberg Raid, 101 Squadron lost 7 Lancasters and crews out of 26 dispatched. In the spring and summer of 1944 101 Squadron attacked targets in France in preparation for and support of the allied invasion of Normandy. These raids were no milk runs and 4 crews were lost on 4th May in the raid on Mailly-le-Camp. On D-Day, the squadron used "ABC" to jam nightfighter controllers to protect the British airborne landings. Returning to targets in Germany by the end of 1944, 101 Squadron Lancasters carried out a series of devastating raids on cities and ports and carried out their last attack on Berchtesgarden on 25th April 1945. During the bomber campaign against Germany 101 Squadron flew on more raids than any other bomber squadron but suffered the highest casualties of any RAF unit in the war, losing 1176 aircrew killed in action.
 

Above details from http://www.raf.mod.uk/rafbrizenorton/aboutus/101squadron.cfm