[citation needed] In 2007, SS501 entered the Japanese market with the maxi single Kokoro, which debuted at number five on the Oricon chart.

[6] During 2001 and 2002, while working as a waiter in a family restaurant located at Jamsil-dong, Kim was introduced to a new set up management company CEO.

[citation needed] Kim Hyung-jun appeared on Ock Joo-hyun's music video, "Catch" in 2004.

[17] The reason was due to Heo Young Saeng's throat condition, which required a surgery, thus resulting in a need for time to fully recover.

[22][23] The group (excepting Heo Young Saeng, who was recovering from the surgery) also lent their voices for the 2006 animated movie Pi's Story.

[43] While both Kim and Park were busy with their schedules, the remaining three members formed 'SS501 Project Group' with Heo Young-saeng as the leader.

[50] SS501 promoted the album with the single "Love Like This", a track collaborated by Steven Lee, Sean Alexander, and Drew Ryan Scott.

[52] Although the release of a new EP was planned for May 1, 2010 (a nod towards "501" in the group's name, or as they call it "501" day), the date was pushed back in order to "add finishing touches".

Because Kim Hyun-joong was still recovering from a previous injury, they sang two ballads instead of the rumored comeback track.

[54] The lead track, "Love Ya", composed by Steven Lee, is a song accompanied by an orchestra, and blended with a piano melody.

Kim Hyun-joong explained in an interview that, at the time, there wasn't a company that would accept them as a whole, thus the group decided to take the chance to focus on their solo activities.

In October 2010, Kim Hyung-jun announced that SS501 was planning release a new album in 2011[58] but was pushed back, probably because of their busy schedules.

Kim Hyun-joong joined KeyEast after leaving DSP Entertainment and started promoting both as an actor and singer.

He then released his debut solo album Break Down on June 8, 2011 and it exceeded 70,000 pre-ordered copies in ten days.

[61][62] Heo Young-saeng was scheduled to launch his solo singing career on April 28, 2011, but had to postpone because he pulled the ligaments on his right hand during dance practice.

[63] His debut solo mini album, Let It Go was released on May 12, 2011, which features Kim Kyu-jong and Hyuna of girl group 4minute.

[66] Afterwards on December 28, just before the end of the year, Heo released a soundtrack song entitled "The Words On My Lips" for Fermented Family, a Korean drama.

Kim Kyu-jong made his musical debut in Goong: Musical, playing the lead role of Crown Prince Lee Shin, in Kyoto, Japan from June 11 to July 1,[70] and debuted as a solo artist in September 2011 with mini album Turn Me On.

[71] The album features rap by Heo Young-saeng in dance track "My Love", and Yang Jiwon of Spica in Yesterday music video.

However, since there was a change in the conscription law, he was eligible to serve non-active duty as a public service worker.

[73][74] Park Jung-min's comeback single, Not Alone, was originally planned to be released on November 25, 2010, but was pushed back to January 20, 2011.

[78] He also contributed to KBS2's The Princess' Man OST in August, where he sang "Missing You" composed by Park Jung Wook.

[80] On March 8, 2011, Kim finally released his debut solo mini album My Girl[81] with music videos for the two lead tracks "oH!

[84] In October 2011, Kim made his theater debut in the romantic comedy musical Caffeine, where he played as a barista.

[87] Kim Hyun Joong was not able to be in the video at that time because he had a shooting on his variety show, Barefoot Friends.

[88] However, Kim Hyun Joong still talked about his debut on his Korea fanmeeting entitled 2013 KHJ Show - Party People, held at the same day of their anniversary.

[93] September 2, both "SS501 Best Collection" versions ranked on the Oricon Albums Chart at number eleven and thirteen respectively.

[95] On January 17, 2015, SS501's special project group consisting of Young Saeng, Kyu Jong, and Hyung Jun performed on stage together at the Musical & Talk Concert, organized by the Seoul Police Promotional Team, which is where Heo Young Saeng had performed his military duties.

[102] On December 29, 2017, Youngsaeng announced that the group had reunited in Japan (minus Hyung jun, who was still serving military,) to discuss plans for a reunion album in 2018.

[105] On October 2, 2020, Youngsaeng and Kyujong held a performance under the SS501 name in celebration of Chuseok alongside U-Kiss, Narsha, Teen Top, and T-ara.